if you travel from europe, one of the cheapest and most flexible ways (often you can change the dates for free and additional things for a relatively low fee even after departure) to fly around the US is a Visit USA (VUSA) air pass. for example, you might get 10 flights (including 2 coast-to-coast trips) for 1200 US$.
the only prerequisite is a transatlantic ticket, sometimes with the VUSA carrier (or one of its alliance partners)
here is aa site showing up-to-date information:
have fun with your trip