The one I use, the effectiveness of which will depend on your campaign style:
Polymorph into the form of a doppleganger if I am dying (between -1 and -9).
The polymorph gives me an automatic healing (gain hp as if you'd rested for a day) which will stabilize me and may even bring me back into positive. I chose doppleganger because if the enemy is still present, I can try the "I'm not the real PC, he hired me to play him to fool you! Ha!" bluff. Since dopplegangers revert to their true form when they are knocked out, the fact that I 'revert' to that form when I go down is consistent with the bluff, and getting the doppleganger's +4 racial bonus to Bluff helps pull it off.
Even if you're in a game where the doppleganger bluff would never work, using polymorph as a contingency isn't a bad choice. You still get the free healing, and can pick some other form that might help you survive (like a high AC monster).