Hi Melanie,
I regularly transfer money from the UK to the US. Two methods I use for maximum exchange rate benefits are:
1) If I'm in the UK, I simply send endorsed American Express or Mastercard travellers checks (available from any Foreign Exchange place) - I just pre-endorse them and make them payable to me, then post it via next-day FedEx. The only problem with this is the FedEx cost - which is about £27, unless you have a free method of sending it by FedEx - i.e. company account! Only really useful for large amounts! A wire transfer is the alternative, but it can be slower and more costly, and if you had a dollar account you'd still have to transfer between banks.
2) If I'm in the US, I simply use my Abbey National bank card! Taking money out in the US with my UK Abbey card gives me a higher exchange rate than any other method, with no fees! The only drawback is that you're subject to your maximum daily withdrawal amounts, which is usually around £500, but some banks will increase this to £1000 on request.
I had thought about using a dollar account, and my cousin who is currently studying in the US does exactly this, but the only real choice here is Citibank, and I dont like them (used to have an account with them). They charge $20/month for the account if you let it slip below £2000 too, and they give you no interest at all on your money (although no doubt they are busily investing your money!).
Hope this helps!
Thanks
S.