K this is how I do it. Basically I take the players collective hit dice (at lets say full strength). So you got 4 players all level 1. A maximum total of 32 HP's collectivelly if they are all fighters and all roll 8's for starting HPs. As far as monsters are concerned, I can't throw the following at them ( +/- 1D8)
One big guy monster 4HD at the most
5 single HD monsters (there's that plus or minus, you don't want to make it exactly even do you! hence the one extra bad guy)
3 - 2HD monsters.
That's sorta how I look at it. Obviously everyone is slightly different. I just like the idea of there being some odds in there. That's not to say I won't put a higher HD monster in a low level campaign. I mean if there dumb enough to let em try, go ahead... meat....
How do you do it? Do you even worry about it?