There are MANY ways to remove stuck and stubborn nuts/bolts, here are some of the ways that I've found work best.
These methods contain both simple options, and some that use luxurious tools such as an impact gun or welder. Use caution, as some of the methods described can cause damage to your parts and might injure you if you do not take proper safety precaution (read: don’t be stupid).
Step 1: Use some muscle
Try a 6-point wrench or socket on your seized nut/bolt. Start by rocking the bolt by tightening then loosing, this may be all you need to break through the rust. Try and avoid 12-point wrenches and sockets as they likely to slip and strip the bolt head.
Another great tool I have found that is made by Irwin Tools is there Original Locking Wrench. Found here
This tool has been a life saver for me! It has removed the worst brake bleeder screws I have seen! It works on rounded fasteners too. This beast can be tightened so much, that it will WARP the nut! These have been in my box now and will not be leaving any time soon! They have a parrot beak shaped jaw allowing 3 sides of contact without marring up the corners of a hex fastener! Neat, Isn't it??