1st: are you using a grip with one hand over and one under?
2nd: have you tried a hook grip? Wrap your thumb around first, then wrap your first one or two fingers over the thumb.
3rd: use chalk. I've been using chalk for over ten years in gyms that don't allow chalk because I bring a wet towel with me and wipe up my mess. That's usually enough to get you by.
4th: practice, practice, practice. I used to do the 20 rep squat program. In retrospect, a death grip on the bar for 20 reps probably lent itself to a decent grip for deads. www.ironmind.com
has a lot of grip ideas and products.