Whenever you buy a Schraeder tube, snug-up the valve core with a valve core wrench. Get one at the auto parts store if you don't have one. Test the core under water before you mount it to the bike (I stick it down into a cup of water) and have a few extra valve cores in your tool box and carry a couple in your seat bag too. Keep the caps on to keep the valve clean and operating correctly.

I understand that running presta and Schrader valves with the caps on goes against the grain and is (quote) "un-cool" but to heck with that. If you want to keep the valves in good shape run caps. Besides, if any rider looks down upon caps, well, it's plain stupid. Really.