The shift lock micro switch is the most common cause. The brake switch is also in the circuit, and the ignition switch can cause the problem as well. To rule out the igintion switch, the next time you try to come out of park, try wiggling the ignition key and see if that helps the release, if it does then you need the electricial portion of your ignition switch replaced. If the wiggling of the key doesn't work the shift lock micro switch should do the trick, you can verify this by checking the voltage there with the brake depressed. The below link has posts on replacing any of the above items.

The pnp switch.