I have done this in the past by disabling the button, but as you'll see by searching about this issue, with ASP.Net pages, it sometimes causes more problems than it solves if you diable a button.  Validation issues, the actual submit of the form, and any other possible error situation can be hard to deal with for the user if the button is disabled.

I finally found the best article (IMHO) regarding a quick and easy way to deal with the issue on 4GuysFromRolla:

http://www.4guysfromrolla.com/webtech/100406-1.shtml