Having a hell of a time with Heretic. I can find HP and each weapon ammo as 4bytes*1, but it occasionally resets without any outward signs, and I get raped. Wouldn't be too bad if i remembered to save more than once every two levels, and if the saves would properly load. Poorly ported game is poorly ported. I am beyond frustrated with it...
For Doom, find HP, Armor and Chaingun ammo (4bytes*1 if you can't read the above posts) freeze and set to 999. You can stand still and blast at the Demon Lords in the last area without every worrying about dying, even on the hardest difficulty. And the nice thing (at least for me) is that I only had to do it once. The variables weren't reset at each level.
Oh, and 4bytes*1 works for Hexen as well. It is actually pretty short, as you only get to play to the 3rd level. The walkthrough is here http://doom.wikia.com/wiki/Hub_1:_Seven_Portals. You only need to look under the first three headings and it's done before you know it.