I'm not sure this works on all cameras, but it works on my 50D and 5D.
When you shoot your 3/5 or whatever shots for HDR, and after you have set the bracketing stops and are ready to fire, set auto-timer. All 3 shots will be fired in rapid succession hardly allowing any time for things to move - unless it's very windy. You also avoid camera shake. In good light conditions they should complete in well under a second. I'm not sure but I think the mirror stays locked up also, which helps reduce vibration further. Not sure about that though.
Any one else do this? Any other method?