1. Clear all levels to 100% when not on challenges. I can, with great difficulty, bring myself to stop attackign a pack of 4 former sergeants on AoB.
2. Hoard medkits and any powerup greater than a small health globe. I always like to save them until the end of the level. After all, I can *probably* survive with 36% health, right? I am somewhat reasonable when it comes to using small medkits but I am terrible with large medkits, which I generally don't use unless I hit 20% health or less. This has led to many mancubus deaths.
3. Hoarding plasma cells. After I figured out how to clear city of skulls with rocket jump (after you pull lever and after the agony elemental spawns), I found the BFG wasn't really necessary. But I still carry around 5 stacks of cells with almost any build, just in case.
4. Hoarding rockets. I like to save a lot of rockets as insurance for the Wall. But post-wall there is no reason whatsoever to hoard so many. I will typically carry 2-3 stacks around until I end up discarding them for something more useful.
5. Avoiding radar shooting in hell. The thing is, I *hate* blowing up items/powerups as a result of barrel explosions. But I need to do this more, especially to detect Mancubi.