To get the best exchange rate your best bet is to get yourself a Nationwide Visa card and use that for as much of your purchasing as possible (easy in America where everwhere takes visa) - and even use it for your cash withdrawals.
Unlike most of the other cards (check your cards t's & c's!) Nationwide don't charge you a fee for using it abroad - hence you effectively get the commercial exchange rate (which is quite a lot better than the tourist rate) on all your foreign transactions.
This is a bit complicated - but the way credit card charges for international transactions works is:
i. Mastercard and Visa charge an issuer fee: this is 0% inside Europe and 1% outside
ii. The card company charge an optional fee: this is set by the card company, but operated by Visa and Mastercard on their behalf. In most cases the fee is set at 2.75% which usually includes the 1% fee outside Europe as the card company absorbs the cost.
Nationwide does not charge an optional fee and they also absorb the cost of the issuer fee outside of europe. Therefore both inside europe and outside europe you simply get the wholesale Visa exchange rate.
Nationwide is also substantially cheaper than any other for credit card cashwithdrawals at 0.5% per transaction (50p min). And also unlike some others, it gives an interest free period on cash withdrawals, something to watch when withdrawing cash from credit cards.
Just to clarify - I don't have any connection with them, other than as a happy customer with more money to spend on my holidays