Whats the best way to get cheap airfare?

I need to fly from the DC area(dulles airport or bwi) to portland, OR. I can fly any weekend to there, but right now its so expensive to go to portland. I could fly to LA for under 250 dollars, but portland is well over 450 dollars. Why the huge difference? Just because LA gets so much more traffic? Even Seattle is at least 25% cheaper. Where can I get great deals for a flight to portland during the month of july? Do any of those last minute sites actually offer good deals?