Try the websites of United Airlines (www.united.com) or American Airlines (www.aa.com)
Both these sites cover all airlines that fly each route - and you can often choose your seat
Try www.travelocity.co.uk, they search through various airlines to find the cheapest.
www.priceline.com allows you to set your own price (within reason). Careful with that one, once you get your price, you are locked in to buying the tickets.
www.cheaptickets.co.uk is similar to Travelocity.
Try www.opodo.co.uk, just booked a Virgin Atlantic flight with them for August costing �198. If cheap flights are not available on a particular website keep going back every so often and try again because sometimes there can be major price changes.