Hi lahdida,
Well done for thinking ahead. I've seen diabetics prevented from boarding in the past because they had no proof that what they were carrying really was essential medicine.
As well as plenty of spare insulin, you will also need a spare prescription from your doctor. In an emergency if you should need more while in the States you can take the UK prescription to a doctor there and he will give you a local prescription with no problem - but you will have to pay for the prescription and the insulin supplied - a US pharmacy will not accept a UK prescription.
Also get your GP to give you a letter addressed "to whom it may concern" stating that you are diabetic and that it is essential that you are allowed to carry medical supplies in your hand luggage. The airlines will always comply if you have a GP's letter, but they can refuse to let you board if you don't have one.
Finally I suggest that, if you don't already have one, you get a Frio bag. This is designed to be carried in hand luggage and will keep your insulin chilled for up to 24 hours. Don't ever pack your insulin in the hold luggage - it will get smashed to pieces!
Enjoy the trip!