Take a train. The bus is miserable.
If you must take the bus, Greyhound is probably one of your better bets.

See: gtscharter.com, CAshuttlebus.com, greyhound.com, gotobus.com. Hope those help=)

First class on the train is awesome, good food, movies, etc., but not cheap so it will have to be Greyhound if you want cheap.