Storage: I store it horizontal, hanging from the grab loops.
Hot Sun: The boat will be in the back of a hot truck frequently, put it on it's side or flat on the hull?
My fat asss: I'm pushing the limit of the weight range, should I loose some weight?
ditto on what of the Captain says. Walking around rapids in general will also prolong the life of your boat.
Of course there is new technique, I believe they call it the Denver/Boulder solution, which is to wait until the first week of may and throw your new boat on your roof racks and leave it there untouched until September. Problem solved.
I have heard that storing kayaks standing up on end is the best way to reduce any stress on the hull. Hanging it by the grabloops may actually contribute to oil-canning over time, but then again it may not. Definitely boof everything in sight, what fun is boating without boofing?