JP's Nymphing Mud - Mojo Mud.  Soft moldable tungsten putty that is just the best to use.  Simply pinch a piece the size you want and mold it around your leader in a ball and then roll it between your hands to thin and spread it.  If you need a little more, add a little and repeat.  Need a little less, pinch some off and repeat.  When you are done, remove it and add back to the supply.  Since it's reusable, a little lasts forever.  Comes in brown and gray colors.