This question has been clubbed to death already in recent threads:
http://www.flyertalk.com/forum/showthread.php?t=385095
My favorite is ShuttlePark 2 (
www.shuttlepark2.com). On-site airport parking is out of sight now, $22 a day I believe. I don't use it anymore.
If you are coming down from Vancouver, though, why not use the Quick Shuttle bus transfer (
www.quickcoach.com)? It gets you across the border check faster.
Also, flying YVR-SEA might prove cheaper than a two-week parking bill.