You might check into the Green Tortiose Bus, I know they use to run between SF and Seattle but they might have expanded www.greentortoise.net Good Luck
Rideshare on craigslist has people looking for riders to split driving and gas cost or vice versa. They usually list where they are traveling from and where they are going or you can post your trip and people going that way contact you. A step above hitchhiking because you can research who you are riding with first.