It really depends on how much time you have. The best thing you can ever do with a first draft is to hide it away somewhere for about six months and then get it out again and look on it with a completely fresh pair of eyes. However, a more immediate approach would be to tweak it yourself until you feel that it reads as well as you can get it and then share it with friends/ other writers and ask them to critique it for you... it is amazing the feedback you get from other people that you just wouldn't have noticed yourself! What is your piece on and how long is it atm?