After hundreds of thousands of air miles (hundreds of trips for business and pleasure), I can safely say the best value tickets are rarely found by searching on travel booking sites (Expedia, Orbitz, Travelocity, etc).
The best way to find a great deal on your next flight is to use a meta search service. This is a service that doesn't directly book flights but rather that searches dozens of them to show you the best deals.
The best known sites for this type of meta search service are HipMunk (link) and Kayak (link). Each service has strengths so I recommend you check both. I ran a quick test for a flight YUL to LAX leaving Sep 12 and returning Sep 14 (for 1 adult).
Hipmunk offers this easy to read chart of prices vs. time of travel. I also like the Agony sorting on Hipmunk. Agony is a combination of price, duration and the number of stops.
Whereas Kayak uses its search algorithms to recommend whether you should buy a trip now or wait.
There are also some less known sites that may be worth Checking out. MoMondo (link) claims it searches over 650 different sites to find you the best deal. I ran my above query against it and voila. It actually found a cheaper flight than Kayak or Hipmunk.
MoMondo also has a handy graph that charts travel prices before and after your requested date. If your dates are flexible, you may end up saving a little but more.
My recommendation is to use all of the above and find the best deal for you (combination of date, # of stops and price).