Honestly, just use Southwest without the promotion - they're still generally the best deal for most places if you buy far enough in advance (especially considering that you get 2 free checked bags!). They're also just a generally great airline - nice planes and staff, almost always on schedule, don't charge massive fees to change/cancel your flight, etc.
For other airlines,
Cheap Flights, Hotels, Destinations | Bing Travel can show you trends in ticket prices, etc and can be helpful.