Personally I think the best tip I can give anyone is to invest in a good microphone. You will immediately see the difference in your results. Assuming you have decent video-editing software, the next most important thing is to learn the three-point lighting technique. Here are my top ten tips, in case they help (feedback appreciated).

Digital Video and Film

(you need to scroll down the page a bit to see the tips -- I'll move them higher up on the page as soon as I figure out how)