First of all, be assured that it's very rare. (And congratulations! It's a beautiful thing!)
I don't know that you really can do much to prevent it (raising your arms above your head actually doesn't cause it), but here's one thing: if your water breaks before the head is engaged, then I don't care if you're at Times Square. Do find a safe spot, but immediately go down on elbows and knees, and keep your head down and your bottom up. This keeps the cord from prolapsing (a similar problem, much like getting wrapped around the neck, just as dangerous) or, if it *has* prolapsed, it keeps gravity from causing the baby's head to cut off blood to the baby. Don't walk far, just to your car/ambulance where you will again lie down immediately. This will almost certainly save your baby's life in the event of cord prolapse.
Since there are a lot of myths out there about the cord wrapping around the baby's neck, if it were me, I'd google it and get a book from the library that has something like "cord, wrapped" in its index.
One thing to keep in mind is, the cord can get wrapped lightly and doesn't cause any problem, just sits there. At the end of your labor, the dr./midwife/whomever can do a manual check and if it's wrapped, can simply lift it up and over the baby's head so the baby can be born without any dangers of tightening.
Again, what I *do* know is that it's very rare.
Congratulations! By the way, I used a Natal Hypnotherapy cd and it worked marvelously (bought it online). You can always opt to get an epidural or whatever if you find it necessary. I for one was in transition walking around, laughing and hosting my midwives. The contractions hurt a little but were very manageable. No joke, it was incredible. Lots of women who use hypnosis for labor have similar results. Google it. Well worth the $30 or $50 I paid for it. Nice to have a natural birth and not have a lot of pain!!! And your baby is born so alert and can breastfeed more easily.
SPeaking of breastfeeding, it can be a little tricky at first, but you can have a La Leche League volunteer come right to your home or the hospital if you run into any snags. Once you learn a trick or two for handling any issue, it's so easy - especially when you get the hang of side-lying nursing! Then you can sleep through the night while your baby eats. It also means that, in splitting up the childcare responsibilities, yours entails lying there relaxing while your baby eats, and Dad does the dishes, makes meals, cleans the house, changes diapers.... :)
What i did was go to some La Leche League meetings while I was still pregnant. Find them in your area. Very helpful, wonderful support!
Good luck with everything!
L.