Use a set of sheets that are the right fit or slightly bigger, including a fitted for the bottom. You can safety pin the bottom sheet tighter or use a set of suspenders with the alligator clips underneath the bunkend. I find that a twin and a full work great, but the king bunk a king sheet is slightly too big and a queen would be too small. The suspenders work great to keep the king sized sheet nice and tight (not that DH minds, but I do).

We just grab a set of sheets from home.  However DW keeps thinking we have the old camper with 2 doubles, not 1 King and 1 double....so the King is always short sheeted. 

I know they aren't truely a King and a double, I just tuck the extra under the mattress.  When I go solo, I usually just bring a sleeping bag, but that is usually when it is colder.  In the summer time I like the sheets and that is when DW goes.