If you apply your original makeup with care, there's no reason you should need to reapply more than once a day (excluding lipstick or gloss, of course). Applying more often will just clog your pores and give you that "makeup mask" look.
Before you even think of adding foundation or powder, your first step should be face-blotting papers. By using these sheets, you'll remove any excess oil from your skin without removing your foundation. Once you've blotted, if you need to, you can smooth out foundation using a sponge. You can add foundation, but only to areas that really need it; you don't want to look cakey or dirty. The same rule applies for blush -- only reapply if absolutely necessary or it will end up looking clownish instead of natural. Then dust your entire face with a light layer of translucent powder to re-set your makeup and remove excess oil. But don't overdo it -- a little shine looks natural.
If you're in a hurry, skip the powders and move straight to the eyes, since they're one of the more noticeable areas of your face. Use a cotton swab or tissue to wipe away any creased shadow or smudged eyeliner, especially on the lower lids, where smearing is more likely. If you want to retouch eyeliner, apply a powder shadow using a damp angled brush instead of a pencil. Only apply more eye shadow as needed to even out the color. You shouldn't need to reapply mascara since it will likely clump and flake off, but if necessary, stop at one coat.
Next step: the lips -- another crucial area for retouches. This is the one area that, no matter what tricks you use, will have to be touched up at least once. Keep lips hydrated with lip balm, and reapply lipstick or gloss as needed, especially after meals. If you use a lipstick made for all-day wear, in essence you should only have to reapply gloss.
These reapplication tips will work great if you your cosmetics have been properly cared for, but if your makeup has gone bad, there's no telling if or how long it might stay put. Read on to find out how to tell when it's time to toss your products.