Basic Nail Care
Reducing Nail Length
There are three basic ways to shorten your nails: cutting with clippers, cutting with nail scissors or filing your nails. The consensus about nail cutting generally is: don’t! Use a nail file, or series of nail files, instead. Unless your nails are very thick (most applicable to your toenails) using clippers or scissors damages the edge significantly and leaves rough edges that should be filed down anyway. If you choose to cut your nails, don’t do it when the nail is wet. When nails become wet they swell, become very flexible and weak – flexible like wet paper, not rubbery and resilient. These can cause peeling and breakage, especially starting near nail edges, over time. Nails should be two hours dry before you clip, cut, or file them, or do anything else to them (polish).
Filing
All filing should be done on dry nails and in a unidirectional manner – from the side of the nail in toward the center, for optimal nail strength. Filing with polish ON is a good idea for very weak nails. Filing technique demonstration: http://www.youtube.com/watch?v=7RrfWakB-lI Avoid filing the sides of your nails since this weakens them.
There are three types of nail files that can be used to smooth, even, shape or reduce the length of nails: metal files, paper backed files of various sorts (akin to sandpaper), and glass files. Metal files are the cheapest, longest lasting and by far the roughest and worst for your nails. Paper files come in many grades, just like sandpaper, and you should probably own a rough one (for reducing length), a medium one (for shaping nails), and a couple of fine grade ones (for reducing edge roughness). Lastly, glass files are very gentle, last for a long time (a few brands are not true glass files – Revlon’s, Ulta’s – and should be avoided because they wear out in a few uses) and tend to give good results unless your nails are very thick. They can however break, and there is only one grade available in most glass file brands so people with thicker nails can find them too slow for sole use.
File your nails into various nail shapes: 
Some shapes actually last longer on various people (depending on nail hardness & use of the nail), but mostly this is aesthetic.
Conditioning Skin & Cuticles
The skin on your hands gets an incredible amount of wear on it and tends to be the driest skin on your body. Dryness affects resilience and health, and using a moisturizer on your hands daily is generally a good idea. The skin around your nails, especially the top of the nail – the cuticle area – affects the health of your nail, since the nail bed at the very top is alive. If you use nail polish this is doubly important, since polish removers dry out the skin around your nail. Moisturizers around your nail, on your cuticle, and even *under* your nails affects nail flexibility, peeling, whether or not you get hangnails and general finger health. This is a fairly important part of nail care; consider buying a good hand moisturizer and/or a specialized cuticle cream (such as Lush’s Lemony Flutter – or make your own!) and use it at least once a day, if not more.
Nail Health
Diet and genetics play a large part in nail health – but not as much as actually taking regular, good care of them! Your nails however are windows into your general health, and where vitamin deficiencies, anemia, and other health issues crop up first. A wavy nail might indicate a vitamin deficiency or an injury for example. Getting regular sleep, eating balanced meals, possibly taking vitamin supplements if you have an erratic eating schedule and drinking enough liquids affect your nail health too. There is one specific supplement that will affect nail health and length more than anything else: biotin (Vitamin B7). Biotin has been shown to dramatically increase nail length and thickness, and also affects hair growth and quality – but some people find that it also affects facial hair growth! 
There are also several categories of ‘nail strengtheners‘ that will help improve the quality of your nail, especially if you have weak, peeling nails, but less so if you have thick and brittle nails. Some of these strengtheners contain components that physically cross link keratin in your nail (‘formaldehyde’ – a.k.a. formalin* based ones), some only overlay a matrix of additional protein on the nail, and some contain Fluoride or Dimethyl Urea to strengthen the nail. A great rundown on strengtheners and how they behave can be found at Loodie Loodie’s blog: http://loodieloodieloodie.blogspot….ers-1-of-7.html (You CAN over-harden a weak and peeling nail, especially with strengtheners that contain formaldehyde, so ymmv). *Please see the Q&A on 3-Free for a bit about formalin/formaldehyde.
Polishing
Applying nail polish takes practice, and your technique affects the life of your manicure. You can achieve salon quality results over time however, so do not give up! There are a few things that you can keep in mind that will help right away, however:
Polishing should be done on dry, oil free (swipe with remover or alcohol) nails that have smooth edges, ideally with cuticles that are pushed back a bit. Here is a technique that helps apply polish more easily (and avoid spilling them into your cuticles): http://www.youtube.com/watch?v=q53GuwSajE8 Some brands of nail polish are shoddy, and they will spill everywhere, leave brush marks, streak, stain – so manicures are also dependent on having a decent formula in the polish.
Consider “wrapping” or painting the edges of your fingernails when you apply polish. That is, paint over the free edge and swipe the brush sideways, getting a bit of the polish on the underside. Doing this with base coats, all polish layers, and your top coat (if any) increases the lifespan of your manicure and helps avoid chipping. Some (most quick drying) top coats do much, much better when your tips are wrapped, since they contain cellulose acetate butyrate, which dries to a hard finish and can sometimes ‘pull’ or shrink the layers under the top coat that do not have this ingredient.
ALWAYS consider using a base coat, or at least a clear coat under your polish. It helps extend polish life, adhere it to your nail better, but more importantly some polish stains your nails! Not all base coats are created equally in this regard. The two main options you have for base coats are: sticky ones (dry matte, and are sticky due to rubberized formulas, such as Orly Bonder, CND Stickey, China Glaze Adhesion) and normal ones (nail treatments, such as OPI Nail Envy, Nailtek Foundation II, Nailtiques, Gelous often double as base coats ). Everyone has a preference and a brand that works best for them. A note about base coats: the Seche brand, while having a great top coat, tends to have fairly shoddy base coat formulas.
The #1 secret to getting perfect looking nails is to use a small liner brush, or an eyeshadow brush in 100% acetone and swiping it around your cuticle and any skin that has polish on it right after you apply polish. Consider applying polish with your left hand (so right pinky to forefinger first) and then your right hand, with your thumbs last (you can wipe off smudges with thumbnails).
Don’t use polish because it takes so long? There are quick dry top coats that are a godsend for most of us: Seche Vite is the gold standard for this and the one you should try first. Most of us use a quick drying top coat and they harden polish in 1-2 minutes, with full, rock-hard cures in 30minutes-1hour. The above brand, while it dries super fast and to a rock hard glossy finish, does contain toluene, while most other quick dry top coats don’t, so that is the one caveat! Other good options for brands to try are: Poshe, Out the Door, Sally Hansen Insta-dri, Essie Good to Go, Nubar Diamont, China Glaze Fast Forward. The alternative to fast dry top coats are “drying drops” such as OPI dry drops, which are thin liquids that apply using a dropper to your polished nails and then let them drop off. They can be messy, a bit slower, but work differently (they contain silicone and a plasticizer) and can be a good option for people that have issues with fast dry top coats – or you can use both!
Removal should be done with polish remover (a lot of us like 100% acetone, or preferably acetone+glycerine – check out the homemade Zoya Remove+ clone that is acetone+glycerine here: http://loodieloodieloodie.blogspot….-plus-video.htm) and never scraped off. Apply some to a pad or cotton ball, place it on your nail for 15-30 seconds and then swipe *down* your nail to remove the color. That way you don’t end up with bright blue fingers all day. For cremes or thinner polishes non-acetone removers are fine, but since they require longer contact with your nail and fingers they can actually be more drying and damaging than acetone or acetone based removers. For glitters, check out the ‘foil removal method’: http://berrypolished.com/removing-g…he-foil-method/
A Word on Nail Salons
If you want manicured nails, learning to do your own nails not only saves you money (although you wouldn’t know it from the collections posted in this thread) but can also be safer. The quality of nail salons varies quite a bit, and if you choose to go to a nail salon please make sure it is a clean one that sterilizes their tools, does not cut cuticles or skin, uses disposable tools where appropriate, lets you bring your own polishes and ideally knows more about nails than you do. Practice and get good at doing your nails yourself, and you can save yourself from infection and from spending money on a manicure that will chip in a week.
Speaking of salons, even the best salon can make mistakes, and even perfectly applied manicures can lead to trouble, especially in the case of acrylic nails or non ‘soak-off’ gel nails. These types of nail coatings or false applications use harsh glues to adhere acrylic to your nails, require sanding and buffing of your nail before application (weakening them), require sanding off to remove the nail, require ‘fills’ to maintain – where your nail growth near your cuticle needs to be maintained, and worst of all: when they get wet, over time, they can lift at the edges and breed fungus or bacteria under them. Most nail salons do have strains of fungus floating around, not because of unhygienic procedures but because customers with acrylics and permanent nails grow them under their nails and bring them in to share regularly.
The ‘soak off’ UV gel nail systems, like CND’s Shellac and Harmony’s Gelish, are a different matter. They can be removed with acetone (at home even), are made of different materials and seem to be safer for your nail and skin health. You can also do these at home, if you purchase an appropriate UV light (or newer systems that use LED lights preferably) for home use.