Cut across an old inner tube somewhere, then cut it again about 5/8"--3/4" away so you have a nice "rubber band" the diameter of the tube. This works best for a 700-25 size tube, as bigger ones might be too long.
Then use it slipped over the curved part of the drop bars and the brake lever, adjusting as needed to clamp the brake firmly enough to keep the bike from rolling. The tension can also be adjusted by how wide the rubber band is cut.
Store it over the top of the steerer tube/stem when riding, hooking it over the stem clamp bolts...you will figure it out.
Also works on flat bar bikes with 'regular' brake levers.
Note: I did not figure this out myself, I saw it in another thread somewhere on BF.