I use wire fence posts. The kind with notches running the leangth of it. I hammered two in the ground about 4 feet apart then I lashed a piece of 2x2 across the top to help keep it from sagging inward.
Next I hung the bag by tying the corners with cord. When I don't want the bag i just untie one side and swing it out of the way. The cross beam also comes in handy for hanging targets like nerf balls and ballons.
You can kind iof see what I'm talking about in this photo I took of my buddy.
Also comes in handy with my cheapo hand cut cardboard targets.