Whenever I'm playing a game and am given a "squad" of people to work with - Halo's Marines, for example - I have a tendency to do everything in my power to keep all of them alive. It gets to the point on occasion where I'll restart a section if one of them dies, especially if I'm driving around in a vehicle and they're riding in it.
Like a lot of other people, I reload my gun after every kill. Even in the middle of fights against a ton of people, more often than not I'll smash the reload button after the first kill out of habit and end up dying because of it.
I ration my health pots, mana pots, and especially the buff elixirs in RPGs ridiculously. In KOTOR I saved all of my mines until the the fights against Malak and Kreia (respectively), then placed all of them at the entrance and lured them in to the mines. Worked every time. Same deal with buff potions and such whenever I'm in those things. I don't use health pots unless I'm 100% sure I'm going to die during a fight, and even then I often will just go ahead and die so I don't waste them. In games like Assassin's Creed I do the same thing with my projectile weapons and often end up simply not using them ever. Brotherhood changed that a bit, since the loot system means I can get everything I use back after a single fight.
In TES games I save almost everything, then use the excess to decorate my houses and such. In Morrowind I killed everyone at one of the plantations, took it over, and used it as my house to great effect. I even brought a bunch of the escort-side-mission NPCs to the place and just left them there. The place was awesome, and the habit only got worse in Oblivion and Skyrim. I spend half of my time playing the game just decorating.
In Minecraft I'll organize my inventory every time I pick something up, and everything has to be in a specific slot (in order from left to right: Pick, sword, other tools (if I'm using them), scaffolding blocks (usually dirt), 4 slots for building blocks (depending on tool load), then torches in the final slot).
There are more, but those are the ones that come to mind off the top of my head.