Book ALL flights for a US trip on one ticket
A lot of travelers don't know this but if you are flying domestically in the United States, often you can't pay for baggage UNTIL you get to the airport. In Australia it is either included in your ticket or you can pre-pay a heavily reduced fee to add your bag at checkout. (That's the purchase checkout not the airport check-in).
However, in the US it isn't unusual to book a flight and not ever get asked if you want to add a bag. I've done this a few times and because I didn't get prompted I didn't realise I had a flight with no check-in baggage. Of course then you get to the check-in at the airport and they want to charge you $40 for your bag and it all turns to crap!
Solution: Book all your flights to and inside the US on one ticket and all the airlines will carry your baggage because you are traveling on an international ticket and they have to provide you with baggage on all international flight tickets. NOICE!
(That's not misspelled – it's an Australian thing. Say it like noise but with the “ce” from nice at the end and accentuate the “oi” sound in the middle.)
You can either do this through a travel agent, groan I know, but trust me the baggage fees you will save will make it worth it. or Expedia allows you to add multiple tickets to a booking which achieves the same outcome.
The internal flights MUST be on the same international booking though or it won't work.
Brad booked his own flights to the US for a boat show last year for him and Mark and they had only 2 internal flights but they both had 2 bags, and it was $160USD in extra bags!