Here's the best way I've found and it works like a charm.
Use a brake line wrench like the guys described above - or better yet just cut the brake line or hose and replace it so you can put a box end wrench on the fitting.
Here's the important part: Don't try to slowly turn it out. Once you have the wrench engaged well on the fitting, give it a sharp pull - sometimes I even hit the wrench with a rubber mallet. It's important that it be a really sharp hit, not a slow turn-out. When you do it slowly it's probably going to round off the fitting. It works really, really well and you're a lot less likely to round it off.
