with PB Blaster or the like soak the threads down and work the bolt out. this is important for something like the subframe bolts or the bumper bolts. as you back a rusted bolt out you might get rust into the threads, this will tighten up the bolt and cause you to possible break it. so you need to work the bolt like you where tapping a whole, run it out a few turns then run it back in while using plenty of PB blaster, this way you rid the rust from the thread but you don't mess the thread up or break the bolt and it will come out, just takes some time
