Do you know what your bank charges for exchange and whether you can use your card to withdraw cash in the states at ATMS and to what amount in one transaction? I found when I was there the bank fee from my bank to withdraw the case was less than the commission rates and exchange rate better. But this was 2009, so things may have changed.
Might be worth your while checking out the Transfermate site. they are advertising ( under personal transfers) a ATM card which you can preload with dollars, using Transfermate's usually very good rates, and use during your USA trip.
Check out both cost of card, any cost of ATM withdrawls , and exchange rate offered, and compare with alternatives.
Another good site to look at is FairFx these have low charges. Alternatively if you frequently visit the US you might want to look at an offshore dollar account.
I'm in the same boat and starting to look now, as rates don't look like they're getting much better. I got some from the PO during the week, rate of 1.21. The bank (UB) were giving the same rate and they would have charged a few quid commission.