I use one of two systems, depending on game style:
For standard D&D, I give fixed hit points per level, depending on class. I use above-average values to improve the chances of PCs to survive. Characters get maximum hit points at 1st level, and if a character multi-classes, taking the first level of a class with a higher hit die, they gain extra hit points to make up the difference (this last does not apply to Prestige Classes). The values used are as follows: d4 -> 3, d6 -> 4, d8 -> 6, d10 -> 8 and d12 -> 9.
It's probably worth noting that I've found that this system works well at low and medium levels. When the characters reach about 10th level, it seems to become significantly harder to challenge them than it should be.
For grittier versions of d20 (Babylon 5, Call of Cthulhu, cyberpunk), I will be using a slightly modified version of the hit point rules in B5. Basically, characters will get 4 hit points at 1st level, plus some small number (1 to 3) based on class. At each level, the character will get a small increment, again based on class. The Constitution modifier does not apply in this case, so a 20th level character might have hit points in the mid 60's, as opposed to 200+. Under this variant, a character's chance to stabilise at negative hit points is dependent on Constitution, with high-Con characters being significantly tougher as a result.
This system makes the game
very deadly, so don't use it lightly.
My motives for using these systems are purely to do with personal taste: I like my fantasy heroic and my horror horrific, moods which these systems seem to inspire, and I
really hate random rolls in character management. Mostly because I always roll badly
