Bags. We all got 'em, we all put stuff in 'em, but I'll bet that each and every one of us has a slightly different way of organizing and managing our inventories. Personally, I find that I'm a "less is more" kind of guy, probably because I get a bit overwhelmed with seemingly innumerable slots full of items both usable and junk-worthy. I like to make frequent trips to the Inn and sell off anything that I don't think I will use again or that I can't auction for a decent price. Seeing as I'm absolutely lousy at working the AH for any kind of profit, I vendor a LOT of things.
Back in the day, I used to try to keep my hearthstone and any important quest items in my last bag, along with a few food and drink items. This way, I could just open my backpack and say, "Oh look, stuff I looted that I need to vendor." Now that I'm 60 and raiding, I've found that my bag slots are taken up with different types of situational items - a few trinkets, my Nature Resist gear for AQ40, my Fire Resist gear for BWL, and a couple of different weapons for whether I'm standing back and need buffs, or in the mix and need to do damage. Popping open bags and trying to find what's what became a bit of a chore, and so I started looking for tools that'd help me manage my inventory.