Let the vibrations ease off for a few seconds before you start
the timer. Don't touch the enlarger or anything connected to it
while exposing. There should not be a reason to touch anything
during the exposure anyway.  Another source of loss of sharpness
is negative pop. During long exposures the negative gets hot and
pops in the opposite direction. If that happens, you need to let
the set heat up past the popping point and then expose.