The simplest and cheapest is to use a rubber band, but I had trouble with the rubber bands breaking. The best solution I found (after experimenting with the options shown in the picture) turned out to be adding a tighter hook that grabs on the clothesline.
Step 1: Materials
2) 19 gauge or heavier stainless steel wire. Lower gauge numbers are heavier wire. It should be stainless, again so it won't rust and stain clothes.