How do I separate hydraulic disc brake pads after pulling the brake while the wheel is off? I take the front wheel off my mountain bike to get it in my car. When I get it out and try to put the front wheel back on, the space between the brake pads isn't wide enough for the rotor. Whoops! I must have pulled the lever on my hydraulic brakes.

How can I separate the the pads without damaging them?
As mentioned try a blunt bladed instrument (A large flat blade screwdriver will do) to pry the pads back in. Just put the screwdriver between the pads and lever the pads apart evenly! 
Just be carful though as sometimes the pistons that push the pads out can come out too far resulting in the pistons being slighly out of line, however if the pads have plenty of material left on them you should be fine to just lever them back in with the screwdriver, ther will be some resistance as you will also be pushing against the pistons and fluid pressure.