When manicurists are training, they have to get one detail in particular right: They must be able to paint the nail in three strokes only, one each on either side of the nail, then one down the middle to finish the job.
If you have a good quality polish, like this Napoleon Perdis Nail Polish in Orange Sherbert ($19), you should only need two coats. Dip the brush in, smooth any excess off as you pull the brush out of the bottle and swipe in one smooth stroke down the length of the nail. Too little polish and the brush won't glide along your nail easily enough; too much and you'll be left with a runny blob of paint on your nail. The key? The brush should look full, but not be dripping, once you've taken it out of the bottle.