We have a house rule where he have bloodied hit points (BHP) and we use DR when you're BHP are hit. This boost is most noticeable at first level the most. So at first level you have:
HP as normal + BHP. BHP = (CON mod + STR mod)x size (1 for Medium)
When HP = 0, damage comes off BHP - DR from Armor (Armor DR = armor AC - 10)
When BHP = 0, you die.
BHP do not increase with level, it is always: (CON mod + STR mod)x size (1 for Medium)
This also makes armor more important and provides an incentive to spend money on better armor, which helps with "improvement" while not actually increasing in level.
EDIT: I am sure one could do it by being cautious or going easy on the PCs, but that is not my style.