Greyhound, alas, has the most frequent service.

Other than that, there's Amtrak and Capitol Corridor train service -- you typically have to take an Amtrak bus or BART to the east bay to catch them.

These just get you to downtown Sacramento, then you have to take a shuttle to the airport, which isn't that cheap.

Finally, there is at least one shuttle service from SFO/SF. Take a look at http://www.shuttlecalifornia.net/

You can sometimes add the SFO-SMF leg to a trip for not too much.