Make your own spill-proof cup
If you buy those single serving plastic drink bottles (Odwalla, for example), use a pocket knife to punch a hole in the lid, insert a straw, and you have an instant spill-resistant drink container.
If you don't carry a pocket knife or other handy hole-puncher, take a lid home, punch it there, then carry it around with you for next time.
Sticking a straw through the hole in a to-go coffee cup lid also works well to prevent spilled drinks.
Related: Sippy cup on-the-go