Mixed grip will allow you to be able to pull more weight. (vs the convention 2 hands in same direction...hands can not slip off as easily due to weak grip strength). There is also something called a hook grip that powerlifters use. Here's an example, http://www.e-lacrosse.com/2005/yeager/pl12.jpg
I havent got used to it yet...but that should help in stronger pulls like cleans and snatch.
Personally I use the mixed grip with Deadlifts, as my grip will go before my leg/back will.
If you need to work on grip, then it's something obviously that only gets better with practice. You can practice just holding heavy DBs, heavy BB, farmers walk (walk with heavy DBs), hang on the pullup bar for time, pinch plates...etc. The key is to practice it or it will not get better. So throw away the wraps, gloves and all those things...just use your hands and some chaulk and you will have a strong grip in no time.