Carrie's tip for spill-proof drinks in the car once the kids are past sippy cups:
I keep empty sport bottles in the car for young kids. Should we decide to buy a drink from a drive-through (my daughters love the apple juice slushes at Sonic), I can order a large one more cheaply, then transfer it into the non-spilling sports bottles before I pass them into the back seat. Saves a little money and a lot of mess!
If you're looking for something washable, there are plenty of stylish, kid-friendly sport bottles on the market. There are the ever-popular Sigg spillproof bottles, and these stainless steel Nathan bottles have a nifty flip-straw. If you're looking for something a little lighter weight, check out these BPA-free plastic bottles from BioGreen and Nalgene.