Does anyone have any advice on finding cheap airfare?

My departure date is Oct 20 and i will be returning the next day Oct 21. It doesnt matter what time i leave on Oct 20, but i would like to leave as late as possible Oct 21st to get back in to Chicago by 6am monday, Oct 22nd. All of the flights i have found, start around $406. I know im starting out pretty late with the search but I decided to go last minute. I'll be traveling from Chicago (any airport) to Seattle-Tacoma airport. If at all possible i would like to leave and arrive back at the same airport. Thanks for the help in advance and i look forward to alot of responses, hopefully!!!