It's no secret that our inventory can get cluttered. I discovered this over the weekend. Rocky sort of prides himself on keeping a tidy inventory with a folder for all the right stuff. I spent a good few hours of filing and weeding my inventory and then realized my big problem. When I would rez stuff, instead of "deleting" objects, I got into the bad habit of "taking" them back. Of course it depends on what the object is and if it is copiable.
Another tip would be to spend time filing the stuff you acquired before you log off. Do it while it's still sitting in your "recent items" list. This makes it easy to find before it gets lumped in the big list.
You may also want to decline the automatic LM offers that you get when you visit places. Wait until you look around and then decide to create an LM for the location.
Duplicate notecards also can fill up your inventory. Read them and then delete them if you don't need to refer to them later.
The photo album, snapshots and texture folders also get messy. If you take snapshots inworld, you know the crazy way SL names them for the location. Rename them to something that is more meaningful. I like to use a chronological method followed by a good tag for searching. Something like 2009-04-06 (theme party at jakes).
I hope these tips help. If you come up with others that you want to share, be sure to leave me a comment.