I'm from California, so thats where I make all my flights to when I go home from the UK.
I usually find the best deals on statravel.co.uk Some fares are only for under 26's but a lot of the time. certainly on the UK version of the site, they are open to the general public (unlike to US site which is specifically for students) Ebookers, last minute.com and expedia are worth a look too. During November/December I have gotten flights on Virgin for as little as £280-£320 which is a great bargain, I have had the best experience flying with them direct to san fran or los angeles. I have also flown American Airlines and British Airways to Cali which was fine, stopping over in Chicago and Texas. I flew once with United, and well that was an interesting experience to say the least1 But Im always going for the best deal so I suppose if they were the cheapest I'd go with them, reluctatnly.
My best tip for transport when you are in the California. If you need to get from so cal to nor cal or vice versa- book a chep flight on southwest airlines. They fly to most cali airports, big and small (not 100 percent sure they go to san fran, but deffo LAX)
But if you book in advance you can get a round trip from LAX to Sacramento (1 1/2 hours from san fran) for as little as 110 dollars. Can't beat that, and the flights only about an hour or so long. MUCH better then driving, I have driven the state more then a few times, and being born and raised in California, I can say the drivers are more then a bit mad out there.
Good luck on your hunt for a cheap flight!