Really, as much as the Americans will probably get annoyed, my advice? Take your time for the first few days!
I drove for my first time over there in May this year and it took me a good few days to take in the different things. Wrong side of the road, automatic gears, turning right on red lights if clear, overtaking on both sides on the 'freeway' and just generally a different feel of driving.
But, honestly, once I'd gotten a few days under my belt I felt great and my confidence grew lots. I'd most certainly not hesitate to drive over there again on my next visit! Don't think I could do without a car now!!