Armor-All Tire Shine
Edit, 8/31/08: Actually, Armor-All Tire Shine may leave your tires looking shiny for a little while, but can be harmful to the rubber.
When seeking a tire shine product, find one that has no solvents or VOC's (look at your tire shine, does it have acetone in it? Think about that!). These chemicals accelerate the process of drying out your tire, sucking the oils out of the rubber. Instead, seek out products like Michelin Tire Shine, which contains no chemicals that would be harmful to your tires. There are others out there (Optimum, Wolfgang), but the Michelin seems to be the least expensive and most readily available.