YVR to downtown. Not an easy task in my past experiences. I do know there are Limos with flat rates that are available that my family has said they would do if we ever needed to go from YVR to downtown. The price, IIRC, seemed cheaper than what a taxi would cost. I believe the Limo was about $45 one way.
the company we used (LimoJet http://www.limojetgold.com) was actually cheaper for us than taking cabs. There were 5 of us (2 in booster seats) so we couldn't fit in one cab LOL There are always limos and taxis at the airport, so you can either call ahead to book or you can just hop into the first limo or cab in the queue.