I have 1/8" parachute cord from an eye at the top of the mast to the end of the boom, tied w/ an anchor bend to a little clasp like this:
It clips to the end of the boom.
I didn't buy that exact one, but you get the idea. I made the line a little longer than I needed and experimented to get it so it holds the boom as high as is practical without impacting the sail shape, then tied it off at that length. Honestly I never unclasp it except when removing the boom to derig the boat (it's moored), but it's very easy to do so if you wanted. I have noticed no wear on the sail but this is my first season with the boat. I can't imagine dealing with a boom flopping around when the sail isn't rigged yet, or at the end of the day, or out on the water in an emergency when you've had to douse the main for some reason...
As Greenlake said, there's several threads on this: The two common options are a "scissor" type affair or a leg that fits into the rudder gudgeons. Unless you're dropping the mast to motor under bridges or something I'd think a topping lift, adjustable or not, would be much less hassle.
Luke