I double up the rope (always recommended for nylon ropes, and safer anyway with hemp or other natural fibre ropes) , loop it through the tie down point, and run two or three half hitches (three for nylon) close to the tie down point after taking out the slack. I do it in such as way as to be able to pull the loose end of the rope so the whole knot disassembles itself, and unties without having to fumble around loosening the knot to get it undone. I put another half hitch for the loose end, so it won't untie itself inadvertently.
If there's a stiff wind expected, a variation on a truckie's hitch can do the job pretty well.