They always used to come down but over the past few years I've noticed they go UP the later you book. I am often lucky when I go on my own and get one seat as a "cheapy" but never for two or more.

I've found flights can go up or down in price much the same as a holiday. As Anne says for one person you can normally risk it but if there's two or more and the dates have to be specific i'd get it booked.
I've always found http://www.travelsupermarket.com good as you can search 3 departure airports and most major airlines in one go. Often saves me alot of time and money