You know all the noise and spots your photo's have with long exposures? Those increase greatly with heat. So, if you need to do a long exposure, put your camera in the refrigerator (or possibly the freezer, but I'm not sure is that would thicken the internal lubrication too much). But besure to not cool the battery, as it will give out less power the colder it is.
Hmmm... I wonder if some one will invent/make cooling kit's for the expensive digital SLR's, so you can use them for star trail photography?