Its a sure bet that the cost of your plane tickets will go up or down before May. There's no figuring out what the airline industry uses to determine prices.
The best scheme seems to be to start monitoring prices for your travel days so that you know what is a good price. Then, when you see something that looks particularly low, go ahead and book it.
I like using
www.Kayak.com to check the price on several different airlines with one search. You can also signup for regular updates.