You know how you're really not supposed to put nail polish remover in nail polish to extend its life, but you end up doing it anyway when you're in a pickle? While it's not ideal, you can apply a similar theory to mascara by using eye makeup remover instead. Two drops or so of eye makeup remover added to mascara that's on its last legs can temporarily bring life back to this makeup standby. Proceed with caution: if you've had said mascara for longer than three months, for safety's sake, it's probably best if you toss it.
And if you're really in a pinch, you can fake thick lashes sans mascara. First, dot on eyeliner in between the lashes to simulate thickness. (Watch a video tutorial here.) Curl the lashes for a few seconds at the base and then again toward the middle. For an even more eye-opening effect, apply clear mascara if you have it on hand.