Joel Spolsky has a pretty good high level design tips:
http://www.joelonsoftware.com/uibook/chapters/fog0000000057.html
Above that, the biggest thing is just trying something out, making mockups. Maybe initial ones on paper, but at some point just try to put together a GUI without hooking up the code behind. See what you think, try some changes, ask other people for their opinions, and just experiment. Best design technique is to get feedback from people, preferably from the target audience if possible.
