If you subscribe to their catalog, they give you the codes on the back. Same goes with the emails too.
Free ground shipping on bike, frame, or wheels on orders over $30 MJH7150RR, expires 11-23-08.
$15 off on orders over $75, MJH7250RR, expires 11-23-08.
Free ground shipping on orders over $125, MJH450BK, expires 11-16-08. $20 off on orders $125 or more, MJH650BK.
MJH2020NR-free shipping on orders $30 or more, excludes bikes, frames, and wheels.
You cant combine any of these codes, so use the one that would save you the most money.