The easiest and most efficient way between downtown and the airport is the Airporter service ( www.yvrairporter.com ). It runs on a fixed schedule every 15-30 minutes and I have used them many times without any delays or hassles whatsoever.

I reccomend purchasing a return ticket at the airport for significant savings. It will run you C$12 oneway/C$18 return for Adults (C$9/C$17 for Senior Citizens).