Books - Get a barcode scanning app (GoodReads works well) and then you can give away your books, buying them electronically if you ever want to buy them again (you probably won't). Or send your books to http://1dollarscan.com/ etc. If you're feeling lazy, just take a few photos of your whole shelf.
DVDs/CDs - Barcode-scan your DVDs and CDs so you can give them away too.
Clothes - Put clothes you haven't worn for 6 months in a box. If they're still in the box 6 months later, give them away.
Selling - If you value your time, avoid selling on eBay. It's an awful UX uploading photos etc. Give away small items and don't bother selling anything under $X as it won't be worth your effort. ($X depends on how much you value your time.) Make it collection-only if you live in a big city and make it the same time for all items (e.g. "collection only at Monday 8-9pm").
Prevention is better than cure - Be careful whatever you buy. Be careful whatever crap you collect at events. These are the same items that a few months/years later are going to cause you clutter and force you to constantly be making decisions about whether to keep them.
I usually just tell myself : Do I want to put this in a suitcase when I move? Do I want one more thing in my place? Usually that works, after the horrors I've had involving moving.
There are a lot of boxes of my stuff sitting somewhere that will probably all go to the trash when/if I ever fetch them. Being forced to move around changed my view on a lot of things.
Good point. Perhaps one way to reduce hobby related stuff is to do as much as possible in hacker spaces or similar things, so that you don't have to own equipment yourself.
