There's a lot of choice!
Work out what you want to do - for example, will all your calls / texts be to or from the US, or will you also want to make / receive calls from the UK?
If the latter, then you may want something like:
ekit, SIMple Calling Plus SIM card
You can buy this SIM in many US airports on arrival. It offers a "global" number, (a UK +44 792 xxx xxxx number) as well as a US number. This allows your UK callers to call you on the UK number, which is free or low cost for them, and surprisingly seems to be free for you to receive calls while in the US.
Of course you also have the US number to use within the US. The rates are OK, but watch out for the 50c connection charge which can eat up your credit if you've lots of short calls.
I didn't do a lot of homework before buying this, so there may well be much better deals - but it certainly was much cheaper for me than using roaming on my UK mobile (at least £1/minute to make or receive calls, and 35p per text).