I'm a big fan of the string type stops. Tried the "dog bones" - total disaster! They kept getting hung up on the rod guides. And the soft rubber stops fall apart after several uses, especially if you try to move the stop often. Like naturenut says, wet the knot thoroughly before pulling tight on both ends. I clip the ends to about a 1/16" tag, which I leave for better visibility. Have not experienced any problems over many, many seasons of using these stops.
"A voyage in search of knowledge need never abandon the spirit of adventure."