$539 for a direct flight sounds like a pretty damn good deal. You could wait and it might go down a bit, but I doubt it will go down more than a hundred bucks or so. Rising fuel costs, etc. If it was me, I'd go ahead and probably get the damn thing now-- the comfort of having the ticket at a decent price (if not necessarily the lowest) outweighs, in my mind, the somewhat lesser cost that you MIGHT get by waiting. Or you might NOT get.
Now, all this being said? I usually wait til January or so to purchase my tickets.
That's just my $.02, you'll have to figure out what's better for you. Buying airline tickets is an eternal game of Russian Roulette that way.
Just try not to beat yourself up too much if they end up going down in price after you buy them. It's not something you can really help. *shrug*