The Boss bass EQ pedal will work in an emergency, but if you're looking for more of a 3db/oct rolloff (strangle switch style etc) I find that the easiest way to get this is with an LS-2 pedal, a resistor, and a capacitor.

The LS2 conveniently supplies input and output buffering and bypass switching, and has well-documented input/output impedance etc.
Well, it's a thought. Passive circuits are pretty easy to find and play with too. I'm using a blend setup based on this idea. (After trying several high tech alternatives including a commercially made crossover, Big Bottom pedal, etc, I'm back to the RC circuit because it sounds better.)