The bowline knot is useful because it doesn't slip.

How to tie a bowline
Hold the line in your left hand near the end. Make an overhand loop. Take the running end of the line and go through the loop from behind, up and behind and around the standing part and back down through the loop from the front.

In the Boy Scouts, we remembered how to tie this knot with the saying: "The rabbit (the running end of the line) goes out of the hole, around the tree and back down again."