Maximum HP
I like to run moderately heroic games; my players get a biased default ability score array (17, 15, 14, 13, 11, 9) and either roll for HP or take half, whichever is better (so if a fighter gets a 3 on his d10, that's rerolled until it's at least 5+).
For some bizarre and unexplained reason, my players don't like rolling 1d6+4 for their fighter characters, even though that's exactly the same probability, with no rerolls to consider...
You end up with somewhat different HD, effectively:
d4 = d3+1
d6 = d4+2
d8 = d5+3 -- or reroll those d8's if you don't like (d10+6)/2
d10 = d6+4
d12 = d7+5 (yes, well...)
At 11th level, your average (Con 12) wizard has 40 hp; his buddy the (Con 16) barbarian has 110. By this system, the wizard's looking at 50 hp, on average, and the barbarian's on around 140. The balance is still roughly 1:3, so it doesn't devalue Con quite as much as full HP and reduces the (sometimes severe) randomness that bad hit die rolls can produce. Trust me, I know about really bad hit die rolls. My current wizard, in someone else's game, has 11 hp at 7th-level.
I would not necessarily recommend maximizing opposition / NPC HP, however. I think that could quite seriously undermine your party's spellcasters (especially sorcerers).
If you do think your crunchies are getting chewed up too quickly at the moment, consider upping the CR a touch, or maybe giving them +2 hp per HD (to the averages listed), but I wouldn't throw much more than that around, or your fights will likely either a) D-R-A-G, or b) become spell-less swordfests, as all your players retire their sorcerers and wizards to beef up some power-paladins or melee-munchy-combat-clerics. On the other hand, that might be what you're after; you might quite like that idea. I wouldn't.