I'm with BlueJay on this -- it's a good technique with an instrument in hand, but can also be done without an instrument. I used to do this a thousand years ago when I was first learning fingerpicking patterns; also when I was a young flamenco student, my teacher told me to practice my rasgueado (the rhythmic finger roll/strum that flamenco guitarists do) on table tops, my pants leg, and any other surface until it was smooth, even and effortless. Did the same thing in my classical guitar studies when learning the tremelo technique (as heard in Recuerdos de la Alhambra by Albeniz; a standard of the classical guitar repertoire). You can do it while watching TV, reading a book, driving to the office, whatever -- basically any time that you could drum your fingers on something, you can practice a pattern just as well.

The other great thing about developing this muscle memory is that you can then move away from strict patterns, and incorporate melody and harmony lines. Once your hand is accustomed to the pattern, you can start to explore adding in other lines, grace notes, etc., while still keeping the fluid rhythm happening, and being able to revert back to the basic pattern at any time. After a while it's like walking -- once you're comfortable on your feet (post-toddler age), you can step around obstacles, change direction, etc., but still keep that steady left-right motion going, and you don't even fall down! It's a great way to branch out from being strictly an accompanist to being a soloist a la Kottke, Atkins, Travis, etc.