Ditch the dirtAlways make sure your hands and nails are nice and clean. Sometimes oil or dirt will react badly with your nail polish and who doesn't like clean hands?!
Thick layersThe most common thing I always read about bubbling is: let your first coat dry before applying the second. Honestly? I don't have time for that! When you apply thin layers of polish, you should be fine. When your layers of polish are too thick, it can cause bubbling because it scoops up more air than it normally would. If you happen to have an old or poor quality nail polish that is too thick, simply add some thinner and you're good to go. Never ever use acetone or nail polish remover because you might as well throw it out straight away.
Stirred, not shakenShaking your nail polish isn't necessarily bad. In fact, all nail polish needs a good shake every now and then! But maybe not right before applying it. Whenever you shake your bottle of nail polish, air bubbles can get trapped in your polish. Instead of shaking, roll the bottle in your hands.
EnvironmentPainting your nails in a humid room, or during hot weather, can also cause bubbling. When it's hot, air particles spread out far and wide and are easier absorbed by your polish. Nail polish molecules expand as well, making it even easier to let those damn air bubbles in. Heat is a culprit, for sure.
Also, applying nail polish in an air conditioned room, or in front of fans etc, won't help much either.