I've used a Battery Tender for a couple of years on a car I store during the winters. This is what you want. It's not actually a trickle charger.

A trickle charger would constantly push in a half to 2 amps and would eventually overcharge the battery if left on too long. The Battery Tender uses something like 3/4 of an amp but stops charging once it senses that the battery is fully charged. There are several different manufacturers of this type of charger.