CNNMoney.com published an article that gives you tips on 5 small home improvement projects that you can do yourself before your house goes on sale. They rate the projects by skill level required, how much it costs, and how much you'd have to pay a professional to do it.
They note that because it can be difficult to recoup the full cost of the improvements, you should concentrate on what you can do yourself (i.e. without a contractor) and really home in on the rooms that people focus on.
The Wall Street Journal also lists improvements you should avoid when selling your house. This article isn't as detailed or thorough as the CNNMoney.com home improvement article but does provide some insight into common pitfalls.
Many times it will be easier and less expensive to offer your prospects a credit towards improvements rather than doing them yourself. Prospects like having their choice of styles (your buyer's taste is something that's difficult to predict) and you won't have to take on the risk of performing or contracting the work.