I use a variant of the DMG variant. Here it is:
When you roll a natural 20, you threaten a critical hit. If you roll a natural 20 on your critical hit roll, you threaten an instant kill. You roll again, and if the roll is high enough that it would ordinarily score a hit, add one to your critical multiplier (x2 = x3). If you roll a third natural 20, the target is instantly killed (reduced to –10 hp). Those immune to critical hits are immune to instant kills.
This requires three natural 20's in a row for an instant kill. Possible, but not too likely. Moreover, if you roll two 20's in a row and then a third roll that would have hit the target, you get a more powerful crit. In the DMG, this would be an instant kill. But in my variant, it only adds one to the crit multiplier. So far it works really well. Have had a few better crits, but no instant kills yet.