1. Wear comfortable shoes tomorrow. This seems obvious, but it's good to remember. After all, if you wear the ones with the heels and step on someone's foot, you'll lose valuable shopping time stopping to make sure they're okay. Soft shoes, people. Slippers, if you can get away with it.
2. Hide your stash before leaving home. Or as much of it as is possible. Nothing ruins a good stash hunt quicker than a spouse wondering aloud if you really need any more yarn. If the thought of hiding it is rather like trying to hide Mt. Rainier, just try camoflaging it a bit. "No, Honey, we've always had that extra bed in the guest room....and it's always been kind of lumpy...why?"
3. Remove all non essentials from your car. You probably need the spare tire and a jack. But a map? Come on. If you can't already find the LYS in your sleep, you have no excuse whatsoever for going on a yarn diet. You'll starve for goodness sake! And you don't need an extra coat or anything....what's warmer than a carful of wool?
4. Forget dressing in layers. If you're carrying your jacket in the warm store, that's valuable yarn space completely wasted. Dress lightly. If you get cold, hug the yarn (don't pretend you don't do that anyway).
5. If you have the LYS memorized (and again, if you don't, what in the world are you doing cutting back on YARN?), it's good to go in with a battle plan. Personally, I'm going straight for the alpaca because everyone knows I'm a big old alpaca whore and I'd have a whole (herd? flock? pack? skein?) of them given half a chance. But work to your own preferences.
6. Pace yourself. Start early and take plenty of time. Working too quickly can result in a bad state of "shopper in the headlights". Sadly, I've fallen prey to this more than once. If you've ever left a yarn store empty armed after falling in love with a dozen things, then you have, too. Just keep repeating to yourself: "Six (or nine, or whatever) month wool desert. Wool, wool, everywhere and not a shred to knit." Or whatever works for you.
7. Be sure to stretch. The last thing you want is to have a perfectly good wool binge cut tragically short by reaching up to a high shelf and pulling a muscle. Shop safe, shop smart.
8. Don't wear anything you've knit. Any other day, absolutely. But you have less than 24 hours to prepare for total wool abstinence. Do you want to spend even one minute of it explaining how you altered or designed the pattern for your sweater? I thought not.
9. Fast calculations can be your friend. Get another knitter to quiz you (or your spouse, if you absolutely don't mind getting that look) with things like "Okay, it's 70% alpaca, 30% wool, DK, needle size six, 98 yards to a ball. How many balls do you need to buy for the cover sweater on the newest Interweave Knits?" or "What colors will your mom absolutely not wear even if you knit them up for her?" You'll want to be able to answer in under 10 seconds for maximum stash enhancement.
10. You'll want energy. A pocket or purse full of snacks is a must. But don't do what I did when I was trying for healthier snacks and forgot to buy sandwich bags. Linty raisins can really take the edge of a lovely yarn feeding frenzy.
11. An itemized list of what you already have is unnecessary. It's yarn. Trust me--it's good.
Good luck to you out there. If I see you, don't be offended if I just nod briskly. Can't talk--yarn binging.