Having family in England, I try to go there each year to visit. This is how I plan my trip.
When I have picked my travel dates, I visit various on-line travel sites, such as Expedia, Kayak, etc., to find the best deals on flights. However, it is also a good idea to double check the prices against the individual airline web sites, such as American, Virgin and British Airways, because sometimes you can get a better deal if you go to them directly.
By the way you will need a current passport! So make sure you get one in plenty of time before your trip (or renew if it is expiring soon) US citizens do not need a special visa for vacations or business trips in the UK if the visit is for less than six months, (although I heard that the Brits are considering changing this time frame to three months some time in the future.)http://www.travel.state.gov/passport/passport_1738.html