Just a reminder that Southwest posted their fares today for Labor Day weekend. They are running web specials for $144 each way from Midway to Los Angeles which is $320 with taxes and fees. That is probably the cheapest round trip you'll find, so you may want to take advantage of it because those fares sell out early.
Southwest is one of the few airlines that doesn't charge for checked luggage, so when booking keep in mind that other airlines will charge you $25 each way for a checked bag. If you have more than one checked bag per person, the second back is like $35 and so on. So Southwest not only has great fares but you don't pay a baggage fee which will save you $50 unless you're planning to take a carry on bag for the weekend.
I also checked the other airlines. They are all running about $320 total as well but you'll have the baggage fee. I checked Expedia and that's for Alaska Air and United. I also check American as well.