If you've made every effort to apply a perfect layer of nail polish and still notice a pesky bubble, there are some culprits -- and fixes -- you should know about:
Beware excess oil and perspiration -- If it's a hot day and your hands are perspiring, you may be adding bubble-building moisture to the polish as you work. To help keep moisture to a minimum, work in a cool room or wait until evening to do your nails next time.
Use a cover up -- Sometimes small bubbles spring up out of nowhere just when you think you have a perfect finish. Instead of starting over from scratch, you may be able to conceal the bubble using a top coat over your polish. The extra layer and enhanced sheen will reduce the appearance of the bubble and may even add some pressure to flatten it out. If you don't have or don't like to use a top coat product, adding an additional layer of polish may do the trick, too.
Thin it away -- Minor bubbles sometimes can be smoothed out using polish thinner. Just brush over the bubbles from the cuticle to the tip of your nail in one or two flowing, continuous swipes, and let the area dry for a few minutes. Reapply polish to the treated spot. This works well with clusters of tiny bubbles.
Use a disguise -- If all else fails, you can always take the easy way out and cover a large, persistent bubble with a nail decal, sticker, jewel or other enhancement.