I've found that using a minimal number of strokes is key. If you can cover your nail with one or two strokes, the layer you put down will be thin, but smooth. When I started painting my nails, this sounded absurd because the brush is so thin and my nails are so broad... but I'm getting better at it.
Stacking smooth layers results in a nice, smooth finish. If you're more haphazard and paint like I did in the beginning, it gets lumpy. In my experience, you can't fix lumpy. You can put a uniform layer on top of a lumpy layer, and it stays lumpy; or you can put a lumpy layer on top of a lumpy layer, and it gets lumpier. Strip the polish off and try again.
One nice trick is to forget about your fingers. If you paint on skin, just leave it be. Once the paint is dry, soak your hands a bit (y'know, shower or wash dishes) and carefully peel the paint away, peeling away from the nail. Or if you're me, just wash your hair and forget about it all... I'm sure some of those bits of polish get stuck in my hair but I never see 'em again.