Removing Torx Bolts:
Packing the head of the torx bolt full of valve grinding compound and then tapping the bit into the bolt with a hammer is also helpful to take up the slop, and prevent the bit from slipping.
The larger torx bolts however can be tougher, and just tend to break bits. One of my favorite tools is my impact driver.
You smack the impact driver with a large hammer, and it drives the bit into the bolt, and twists it at the same time. Usually the shock will help break it loose, while not letting the bit slip. Usually a few good smacks will get the job done. However remember with this method you have to smack the impact driver with a large hammer, this doesn't work too well on smaller things, or body panels.
Typically heat helps a lot. Generally you want to heat the nut side, so it swells, but sometimes you have no choice but to heat the bolt. A propane or mapp gas torch available at any hardware store is sufficent for this purpose. Mapp gas is preferred over propane as it burns much hotter. The heat will also help if the bolt had any loc-tite on it. Becareful what you heat though, especially if you've sprayed penatrating oil on the bolt. It's always a good idea to have a hose ready just in case.
Now when all else fails, or the head strips, it's going to take something more serious. You can try easy outs, which will require drilling a hole in the bolt, I've never had much luck with them though.
However before you try an easy out, you are much better off to try a left-handed drill bit. You have to drill a hole in the bolt anyways to use an easy out, you might as well try it with a left-handed drill bit. You use it like any other drill bit except you spin the drill in reverse. Between the reverse spinning and heat created by the drill bit, many times the bolt will walk itself out while drilling.
Welding a nut to the head is usually where I end up, especially if the head strips. First I take drill bit that will fit in the center of the torx head and just drill it enough to get the paint off the head in a spot. Next I take a nut the size of head of the torx bolt, typically 1/2" nuts work well.
Then I start welding in the center of the torx head, and fill the nut full of weld. You'll need to let the bolt cool before trying to remove it, but the heat will help un-seize the bolt.
Once I get the torx bolts out, they get replaced with regular hex heads. In most cases you'll want to use some anti-seize on the threads to make sure the bolts come out later down the road.