1. Use oil to set your manicure. There's like, real nail oil you can use, but in my obsessive perusing of the internet, I've learned that cooking oil does the trick just as well. Just make sure you're careful with where you're spraying or you'll end up with a fine mist of cooking oil on everything you own.
2. This Nail Rescue Kit is actually one of my favorite things in the entire world. The little cuticle snipper is highly addictive. If I don't have time for a real manicure (which is basically all the time), I like to clean up my cuticles and then moisturize with Egyptian Magic. Top it all off with a fresh coat of polish and it's almost as good as going to the salon.
UO Left Bank polish
3. For fine, straight lines, it's always possible to freehand designs with Nail Art Pens, but if you've got shaky baby hands like me, then using Scotch tape to block off sections is always a better option. Just make sure that you stick the tape on your hand a couple of times to get rid of some of the stickiness before putting it on your nails. The Beauty Dept. has some good examples of taped off manicures.
4. If you're super bad at painting on only your nails, another good trick is to rub some Vaseline or lotion on your cuticles before painting. That way, if you end up smushin' some polish on there, it'll be easier to take off. (I know some people who paint their fingers all willy-nilly and then take a shower to remove the excess nail polish from their cuticles, but that just seems soooo tedious. PRO-TIP FOR YOU.)
5. If you have some nail polish that has thickened over time, DON'T PUT NAIL POLISH REMOVER IN IT! It's a good quick fix if you're antsy, but over time, it'll just make the nail polish even weirder. Instead, pop some clear polish in there. It should thin the polish out enough to use it again and it won't break down over time like nail polish remover would. MAGIC.