I use plastic tubing with 3/16" exterior diameter, cut to the length of the spring, inserted inside the spring. Works really well for dampening the spring rattle. Really easy to put in place and also remove if necessary
Use those foam inserts from pickup packaging. Just stick them under the springs and done. However, you should put straws inside of the springs. They fit perfectly and dampen spring vibration without getting caught between the spring coils.
I use plastic tubing as well, but also recently started putting a thin strip of the foam from a pickup box under the strings just behind the nut. Pretty amazing how much noise comes from the strings there. Now when I make a tight stop its all dead quiet.