I used the following house rule for dying.
If your hit points decrease to a negative number, make a Fortitude save, taking your current hit points as a penalty. If you fail DC 1, you die. If you make DC 11, you stabilize. Otherwise, nothing happens.
On your turn, if you are at negative hit points, make a Fortitude save as above. Results are as above, except that if you make DC 1 and fail DC 11, you lose 1 hit point that doesn't trigger another fort save.
For example, if you have a Fortitude save bonus of +8 and you are dropped to -11 hit points, you roll 1d20-3. On a four or better, you don't die. On a 14 or better, you stabilize.
This means that everyone gets harder to kill as they advance, and the physically tough characters (high con or good fort save) are more likely to survive a wound that knocks them unconscious.
It also prevents "I'm only at -5, so finish of the bad guy; you've got plenty of time to heal me" metagaming, since you might roll poorly the next time you have to make the fort save.
If you have any "use them when you really need it" bonuses, you can apply them to the fort save. I know that somebody wrote spells in 3.0 that would let you roll 2d20 and use the better, or add up to +5 to a save.
Toughness should probably add +3 or so to the save. I didn't think about it, because nobody took it in my game.
My players loved this rule when they were surprised by large earth elementals. I think two of them went into the negative teens and one was at negative twentysomething. When the cleric woke up after the battle, his first question was "Is everybody alive?" He was surprised to hear the answer was yes.
