Just slip a spoon into the small metal hook at the end of your strainer. The spoon will rest against the edge of the pot, anchoring the strainer in place.
Every strainer is a little different, but this hack will work for the vast majority. It works so well that it's almost like strainers were designed to be used this way.
Over 4,500 enthusiastic users took to Reddit to share their excitement for this unexpectedly useful hack, with one user saying, "My. Goodness. Why did I not think of this??"