When travelling from the UK to the US, what are the options for having a cell phone in the US? Do i need a new phone? a new service on my existing phone? or are there other options? from your experience, which option is best? thank you!

I think you should ask this question in the US section not in the Uk section

Whist ribeirasacra is right that you'd be better off on the US forum I can tell you that we used our Dutch phones in the US no problem. We didn't use them extensively as it is expensive, but had them so we were reachable in case of emergency. They came in useful a couple of times for booking restaurants. We didn't use them for booking hotels as we had a laptop with us, and booked via internet.
Most modern mobile pones will work on American networks, but you should check you model to see that it will.

Do i need a new phone?
US uses different frequencies [850/1900] for their GSM service. Do you have a tri/quad band phone? If so, you can roam on your service - visit http://www.gsmworld.com/roaming/gsminfo/index.shtml to identify your cell company’s roaming partners.
a new service on my existing phone?
If your phone is tri/quad band and is also “unlocked”, you could buy a prepaid SIM card from a US company like T-Mobile and use your phone with a US tel # at a cheaper rate than roaming on your UK service.
or are there other options?
You could buy a phone and prepaid service from a US service provider.
which option is best?
Depends on your circumstances and intended use. Keep in mind that texting is usually the cheapest form of communication with those back home.
cheers

You can pick up a prepaid phone, like virgin mobile, for about $30 with airtime almost anywhere.

I agree that the easiest thing to do would be to buy a "pay as you go" phone when you get here. They are available everywhere--Target, Walmart, Kmart, drug stores such as Walgreens and convenience stores such as 7-11.
I don't use a cell phone much, so I have a "pay as you go" Virgin Mobile phone. Other brands to look for besides Virgin Mobile are TracPhone, T-Mobile and AT&T. Phones prices start as little as $10 and minute cards usually start at $20 for 200 minutes.

The pay as you go option seems the best for you.

Is the same true when going to the UK, do they have these pay as you go phones to buy? We normally borrow my husband's Dad's phone (who lives in the UK), but maybe next time we could just get one of those.