This is when it's going to get hard. The reason most attempts at "cutting back" seem to fail is that we present ourselves with opportunities to buy more yarn. Therefore, some rules are in order.
Though this should be obvious, the number one rule is this:
NO MORE BUYING YARN. NO NO NO NO. NO MORE. NO MATTER WHERE YOU FIND IT OR HOW MUCH IT'S ON SALE, EVEN IF IT'S SOCK YARN. I am not making any wimpy-ass allowances for you*, this is the big league now, and you are a grown up, dammit, and you're going to play hardball.
(And if you are not a grown up, please don't read this blog, it uses many no-no words.)
and to that effect....
2. Absolutely, positively, NO going into a yarn store unsupervised! In fact, try to avoid going in at all. I like to sit and knit in my LYS's coffee shop and enjoy a delicious tea latte, but I can always do so with my boyfriend (who also knits, now) in tow, since he will scold me violently if I do anything stupid (though my will power isn't really THAT bad anyway.) Likewise, if you meet up with a knitting group at a shop, tell them all not to let you buy yarn. Tell them firmly. Say "If I buy yarn, please rain down your harshest criticisms upon me". If you absolutely can't resist, leave your credit cards at home. Not in the car, AT HOME. You could walk to the car and we can't risk that. You can have $2 cash for coffee or tea, and just really hope you don't need money for anything else while you're out.
3. Likewise, fiber festivals are a HUGE no-no. I know they are a lot of fun and believe me, when I had to skip OFFF this year, I was pretty sad about it. But the great thing is that they come back again every year! And maybe, by this time next year, you'll be allowed to buy yarn again. Maybe. If you work really hard.
If, for some reason, you absolutely NEED fiber (as opposed to yarn), perhaps you can go with strict supervision...but why would you need fiber except to make more yarn out of? So you don't need it. I promise. Unless you're a wet-felter, or something. Or a vendor. But still, bring a hard-ass friend who will keep you in your place.
4. Yarn websites? Nope. Use the parental controls to block them if you have to. Go so far as to put a password on knitpicks and WEBS. Take any destashing boards off of your Ravelry forum page so you don't see them (though you can still use them to get rid of your own yarn, obviously). Whatever your weakness, make it as difficult as possible to access.
5. Instruct your spouse or family to dispose IMMEDIATELY of any sort of paper correspondence coming from such companies. And you are only allowed to read knitting magazines if you can succesfully ignore the ads. If you ask nicely, maybe a friend will scribble all over the ads with sharpie for you.
Feeling the burn yet? You will. You may need to change the route you use to drive home or find somewhere else to kill time on your lunch break. Sorry, but them's the apples.
*Ok, I am granting one single exception, and it is this:
IF you are making a gift, AND it is for a special occasion (birthday, Christmas, Hannukah, a birth, a wedding, etc... no minor holidays, no made up holidays, no sad excuses to purchase yarn) AND the giftee has requested an object of a particular character (color, or fiber, etc.) that you don't currently have available in your stash, you may purchase only what you need and no more. If they haven't requested anything and you can possibly manage to, please use your stash. If you don't, you are cheating, and cheaters never win, especially in the game of stash elimination. DO YOU HEAR ME SOLDIER?
Also if you start knitting a large project and find that you need one more ball to finish the last 6 inches of the second sleeve, you can go get one more ball. But you have to at least THINK you have enough to start off with... no fair starting a project if you know you'll need more. If you need more than one ball, too bad. Frog it and make something else. You should've thought of that before you cast on. Brutal, yes, but self-discipline should sting a little. And maybe tingle.
How to deal with gifts
If, during any portion of this stash elimination exercise, you should receive an unsolicited gift of yarn, you may accept it to avoid being a jerk. But you must immediately deal with it in the same way you deal with the rest of your stash, by going back to steps 3, 4, 5, and 6. Obviously, you should not ASK for yarn as a gift, and if someone simply OFFERS you the yarn, don't take it unless it's cashmere in your favorite color.