Are you getting blurry shots with long lens at shutter speed ~ 1/40s to 1s even with a tripod? The odds are good that they are caused by the vibrations. One of the good ways of avoiding vibrations is to use mirror lock-up (MLU). Here is an interesting discussion on DPReview. Mirror lock-up made a significant difference in the sharpness of the images. If you are not familiar with the the terminology, there is a very good article on Luminous Landscape you can read.
Not all DSLRs have MLU that can be used for shooting. Many DSLR models can lock up the mirror for sensor cleaning purpose only. Often they offer an alternative called “exposure delay”, which delays the shutter release after mirror going up by a short period of time to avoid the mirror vibration caused blurs. However this may not be as effective as true MLU because the separation of the mirror up and shutter open are too close for the vibrations to complete dissipate.
In addition, I found the following articles about proper long lens techniques:
Added on 8/6/2007: Also check out this test. It convincingly showed the effectiveness of MLU at slow shutter speed.