In addition to a metranome and/or drum machine, BG musicians would do well to listen to the rhythm guitar of the likes of Jimmy Martin and Lester Flatt. I say this especially of the type of rhythm Jimmy Martin. I've picked with far too many people lately that want to play new grass or progressive kinds of music with a grass spin that have NEVER managed to master the powerful rhythm of the bluegrass masters like Jimmy Marin, Lester Flatt, or Bill Monroe. I once heard Tony Rice tell a young guitar player to forget the new grass and fancy grass until he can master the rhythm of Jimmy Martin. Tim Stafford will tell you that, Bryan Sutton, and even Dan Crary, not to mention the masters of other BG instruments like J.D. Crowe, and Doyle Lawson. We have to master the masters then we'll be able to play the progressive stuff well.