I fly over to Houston about 4 times a year, and it seems that the lower fares start showing up about two months before. It doesn't hurt to keep checking once a week or so. From what I have seen, most airlines update the fares once a week,, with continental, which I fly with, they tend to change the fares early in the week. Also, check out flyertalk.com,, some good tips on there for getting decent fares
I hate flying in the Summer months, it's always so expensive. I am flying earlier than usual this year tho, on May 22,, but it's still nearly $1000.
,, normally when I fly over at the end of July, it's about $1200. which just makes me cringe. Still, that's when my girls are out of school, so it's not really like I have much choice.