Add a mostly-spillproof lid to any cup with Glad Press'n Seal wrap
I use Glad Press'n Seal whenever cup lids get misplaced, broken, fed to rabid raccoons, or simply tossed for leaking.
I fill the cup, make sure the outside rim is dry and place a square of wrap over the top. Press around the rim to get a good seal, smooth the excess on cup's sides, poke a small hole (just large enough for a straw) and you have a lidded cup. (I cut the wrap in half to reduce waste and use it to top two cups.)
To refill the cup, I dry the outside, unpeel, refill and reseal. While this doesn't make the cup completely spill-proof, a few drops are easier to deal with than a full cup.
Donna's tip inspired Adrienne (who first edited this hack) to put a roll of Press'n Seal and some straws in the car for a recent family roadtrip. Her follow up:
We found that this approach also works with bottles. A few pre-cut squares and straws in a baggie might come in handy when dealing with open cups at restaurants.
More: Press'n Seal hacks (I love Press'n Seal. I never buy regular plastic wrap anymore.)