Heat gun, acetone, clay bar. If it's real bad, I usually heat up the bulk of it and use a plastic chisel to thin it out a bit by removing the excess. Obviously don't scrape down to the factory paint.
Use the heat gun to soften the tough areas. Remove the excess with acetone.
If it's all up in the wheel wells, I would recommend just using a plastic die, or undercoating instead of trying to remove it. Use the clay bar only when you've gotten the bulk of it removed. The clay will easily pick up those left over stragglers.
Once you've got it all off the paint...tape off around the wheel wells, and spray the wells with the die or undercoating. it will look like it never happened.
Sometimes it's a piece of cake. Other times it's a huge pain in the ass. Depends how bad it is.