Steve Hargreaves at CNNMoney.com has posted a great article outlining many of the factors that have boosted pump prices since 2004. You can find it here. It's obviously US focused (so don't flip when you see the headline "Who's to Blame for $4 Gas"), but with the connection to the commodity markets on both sides of the border, it stands up in Canada too.
It's a good illustration of how many different and complex factors can come together to impact one thing - the price at the pump. And how each year one main factor stands out amongst all others.