I haven't been to OCS but I have been a seasonal mountaineering instructor for the last 4 years: NEVER wear cotton socks, Smart Wools are the best socks I've ever used (they're expensive, but worth it), gold bond powder is a good idea like the above have said, and duct-tape is a great blister prevention product. Put it on your feet where ever you might get blisters. It acts as a second skin and takes the friction that normally causes blisters. I worked with a guy who would cover the bottom of his feet and totally wrap his big toes everymorning before putting on his socks. I put it on the back of my heels. You do have to be careful, it can dry your feet up it you leave it on too long ( a few days or so). Besides, it's a proven wart killer: added benefit.
If you get one, drain it (don't peel it though), cut a "doughnut" of moleskin to go around it and cover it with duct tape. Eventually that soft skin will harden like a calus.
A roll of duct tape too bulky to carry around. Re-roll it onto a broken pencil or pen or even itself, stick a couple of "patches" into your pocket, share with your friends and watch the smiles appear.
Thorough enough?