I've used a variation of the following rule for a number of years, almost since I started DMing... aaalllll the way back in 2nd edition (when the game was still called AD&D!)
Normal Rule: PCs die when their hit points reach -10
House Rule: PCs die when their hit points reach -10 + CON bonus
The upside is that most characters now probably last a round or two longer. Particular weak characters could expire sooner, but at least there's a logical relationship on a PCs heartiness and how long before they die.
Back in second edition, my rule was a bit different. A PC would die at -CON value. It probably had a more beneficial effect, since I recall most of the PCs having a con value in the 12-16 range. Interestingly, years later I found this post on Monte Cook's website with his house rules on death and dying. Seems he uses -CON for death in his 3e games, along with a modification to the unconscious below zero rule.
Lastly, it'll be interesting to see what D&D 4th edition has planned for this most important, yet most feared-by-the-players part of the game!