The first thing you have to do before conquering your stash is decide that you actually WANT to conquer your stash. If you're only trying to cut down because your husband is whiny or because your mother nags you every time she comes to visit, that's not going to cut it. You have to WANT it. You have to TASTE it.
Personally, I don't really think a stash is a problem, as long as (1) you can afford it, (2) you have space for it, (3) it isn't more than you can realistically knit up (in which case you might as well use that money for a good cause or something.) But there are definitely benefits to cutting down.
So first, figure out what you're trying to accomplish. I've got approximately one CD tower + two cedar chests of yarn (one of which is at my parent's house across the country and which I guess will be ignored for the purpose of this destash... I'll have to do it all over again with that batch later,) plus a spacebag full of unspun fiber. I'm not worried about the fiber, it seems like a pretty reasonable amount. It would be nice, however, if my whole yarn stash could fit in that CD tower, all pretty and on display. I could certainly use the extra storage in that chest, since I live in a tiny apartment with a LOT of craft supplies and clothes. Decluttering is a big motivator for me at the moment.
There is also a certain freedom to having very little yarn on hand. It frees you up to start any project you want, without feeling guilty about going out to buy EVEN MORE yarn just because you didn't have the perfect thing in your stash.
You'll have to identify your own reasons. Write them down and stick them somewhere where you'll see them frequently, if you need to.
This is the easy part. Tomorrow, we talk about the rules. Get ready.