If your only option is to use those coin-op air pumps at the gas station, do yourself the favor of taking off all four caps before starting the pump. It's no fun fumbling with those things when you're racing against the clock to fill all your tires in under a couple minutes. There should be a sticker inside the driver-side door that lists the ideal PSI levels of the front/back tires too. The best option is meagre_graeme's suggestion though - having an inflator in the car is great for lots of things!
