This quick trick lets you easily find the center of a rope again when you've untied it, but not coiled it immediately. I make a habit of doing this every time I finish untying a rope, before setting it down.

Most of my rope kits have four different lengths of rope, so I'll sometimes vary the center-keeping knot that I use to help me keep track of which are which if I'm going to re-use them in the same scene. I use the slipped overhand shown above for the rope length I use most, and a non-slipped overhand, figure 8, and slipped figure 8 for the other three.