There are a couple of points that may be of help: turn off IS if you have it. It won't work if the camera is supported in any other way than hand-held, but the motor will still operate causing vibration inside the lens if on a tripod. The other thing is to research you lens' optimum aperture and set it at that. If a lens is shut down to its narrowest aperture diffraction might occur and that could be giving you blur problems too.
If you are a Canon user this article may be of some use to you, but it will be interesting reading to all photographers no matter whos lenses you use:
http://www.usa.canon.com/dlc/control...articleID=1386