The humble Rubber Band. It makes your bike a fairly rigid plain allowing you to easily lean your bike almost anywhere without it toppling over.
There are certain immutable laws of the universe and one is that if i used an elastic, it would snap when I needed it. I use a clothes peg. Squeeze the brake lever and then jam in the clothes peg so the lever won't release.
No photos handy but can take one if someone can't visualize it....