Debt reduction by hopping around?

No, not destroying your credit card statements by scrunching them with all of your weight on one foot.

But seriously, a very good friend of mine recently worked his way completely out of credit card debt.  That's good, but I didn't realize the extent.

Close to $40,000!

It took years to get out of this — a lot of scrimping, a lot of throwing entire bonus checks at the debt.  And also a lot of consolidation, balance transfers, etc., to lower-interest cards, as well as conversations with the credit card companies for better deals.
Credit card companies have to work very hard to acquire new customers.  After all is said and done, (I recall) the amount is north of $100 per customer.  So there are a lot of incentives thrown at prospective customers, as well as incentives to keep them on.  They're not let go of without a fight!

Recently the rage has been “0% interest on balance transfers for X months” — meaning “Come on over here with your debt, and we won't issue any finance charges for a while!”

Plus a lot of mumbling, 5,700 word-per-minute speech in three-micron Swahili Zaph Dingbat type that discloses all of the ways that they can instamagically raise the rate to the default (translate: ridiculously high) interest rate faster than you can say “Plastic Fantastic.”

My friend was able to use these kinds of offers pretty effectively, and a lot of the payments he was making were mostly knocking the principal down near the end.  But the risk was there to get hit hard with interest, say by going over the credit limit or by being late on a payment.  These offers are a mixed blessing: great if you play by the rules, horrible if you don't.

So if you're offered a low-interest-rate balance transfer for your debt, you can end up saving some dough depending on your situation.  But be sure to check these things out:

• Read the fine print of the offer.  All of it!  These are the rules.  If the rules aren't given in the offer, but instead on a website, check them out there.  Highlight anything that affects your interest rate, especially if the action instantiates the default rate.  If you don't understand the rules, find someone who does.
• Run the numbers.  Are there transfer fees?  How long will it take to pay those fees back based on the interest you'll be saving?  Or isn't it worth it to transfer?
• Consider calling up your current provider.  They may lower your rate if they feel you'll be taking your business elsewhere.
• Get organized.  (This is coming from a person who struggles to get organized.  This is also the very reason why I stay away from taking out 0% balance transfer offers, investing the proceeds, and paying it back before the offer goes away.)  Organization will help you follow the rules so that you can keep your low rate.
• Watch for any deadlines.  When does the teaser rate expire?  What does it go to after the expiration?  Have you gotten other offers that you can go to if the post-teaser rate is too high?

Any other gotchas that might throw off this kind of debt reduction plan?