First, make sure you have clippers meant for full body clipping. The clippers you typically buy for trimming bridle paths, whiskers, and fetlock hair usually aren't going to do well for a full body clip unless you do it over a period of a few days. Otherwise, you run the risk of burning out the motor or hurting the horse if the blade gets too hot. It's very important to make sure you have blade cleaner ready and handy, spray lubricant, and sharpened or new blades. I keep 2 of each size handy and I body clip with a #10. I know you can get longer blades, but they work best for me - not too long, not too short. Also, timing can be a factor unless you need to do it now for health reasons.
Smartpak has a good article with good tips (from what I saw). Body Clipping 101 » SmartPak Equine Blog
I do it in the winter for my horse and the difference is so noticeable with his level of comfort and how much more quickly he dries and cools down. I've also seen many Cushings horses clipped in the summer and they're much happier as a result.