i've flown that route many times this year because we have business in both cities. my single best piece of advice for you is to get a status comp from america west asap.

i've never paid more than $144 each way (most fares are one way --> very flexible) and have been upgraded 100% of the time. this summer, they had a $100 fully refundable, no advance, base fare between dca and pdx, which was just awesome. more recently, i've had to use bwi to get the lowest fares, but dca and iad are still reasonable (rarely more than $200).