What's the best way to build a track selector?
Minecraftaddict has a series of tutorials on how to build minecart stations with multiple destinations. The episode included below showcases how you can use RS-NOR latches to change destinations.
The basic principle is that the main destination line must branch off onto various destination tracks, and these tracks are selected by Redstone Torches toggling on and off.
Also, TaviRider came up with an ingenious way to make a track selector for a minecart station. The design is extremely compact, and surprisingly, all the inputs all use the same input line.