We've done a couple things in the past, first is to lock it up to a tree, however if you are somewhere remote or have no trees around that's not a great idea, obviously. The best is probably to put it in a vehicle when you are not around, but again if you have 3-4 guys or more and just one vehicle that isn't going to work. If it's someplace with a local shop sometimes they will allow you to store them there as well.
One thing we've done before is get one of those cheap family room tents that will hold like 6-8 people and put bikes in there. At night someone is going to figure there are people in a tent and probably are not going to wander in there, the same can be said of if you are gone as well. If they do, they will break into a car etc. as well.
It's sad, but I know this year at least one person who was camping near us in the slickrock campground in Moab had stuff stolen from thier campsite. Putting a tarp over it may actually attract people to the bike since they may figure that if it's so nice you put a tarp over it, its probably worth a lot of money.
Bottom line is like anything else, if someone really wants to steal your bike they will find a way. But I think mostly it's doing just enough to make it too much of a pain in the ass to bother with.