It's a slow process, but I've had some success with it. For me, at least, physical relaxation is key -- I do progressive relaxation drills daily. (This is the sort of thing where you lie down and consciously relax each part of your body in turn.) Making sure I have other things to feed my mind -- music, reading material, etc. -- is also crucial. I've got a Stan Kenton jazz record on right now, very mellow, and it's harder to feel stressed and bitter while that's playing.