Assign Hit Dice (d8 for most things, d12 for Dragons and Undead).How about expanding on this idea and assigning the type of HD based on the creature type/size? For example a tiny little bugger like a ratling would use d4. Humanoid creatures like goblins use d6. Larger man sized creatures use d8. Larger than man-sized or very tough creatures like the undead would use d10 or d12 and finally huge creatures like giants or dragons could use a d20 (or even d24 or 2d10?). As an extra feature - this HD dice type could double as both hp base and their attack damage. Doing this you would merely decide the size or toughness of the creature and then how many HD you wanted to give them for bonuses and the HD and damage would be pretty easy to follow up with. It's pretty easy to figure out the average of each die type and use that as the HP baseline. You could have very skillful tiny monsters that were 7-8 HD with associated to hit bonuses but with low HP and damage due to the d4 base and you could do large lumbering tough monsters with only 1-2 HD but using d12 or d20 for hp and damage. I think it would make things easier to remember and allow you to tweak things pretty quickly for an encounter by whipping up in extra skillful goblins or slow clumsy giants simply by changing the number of HD. I wouldn't want to have too many categories and types of dice but a simple size/toughness based chart should suffice and it seems flexible enough to handle oddities.
I'm going to do some number crunching to see how it works out but this idea seems like it may have legs. It certainly will clean up the monster section for me so I have hopes it will work out. It seems like such a simple idea is someone doing this already?