I'm not sure about the Houston area specifically, but I have some general advice for you, as I have also tried meeting them all of the four times I've seen them so far (but have yet succeeded) =(
First off, it depends on where they need to be the next day. If they have a concert the next night, in another city, you can almost guarantee the band members will be long gone before the concert goers even get out to the parking lot. This is what I was told by a security guard back in 2008. So this limits the chance of meeting them to before the concert, only. If they don't have to be somewhere the next night, there is a good chance they could be staying in your city for the night (or there's also a good chance it could be reversed, where they travel first, and then hang out in the next city until the next concert). When we were in Chicago a month ago, we had stopped at a gas station that was right outside the venue afterwards, and we saw Alice Cooper in a limousine parked at a hotel that was right behind the gas station. So they have to stay "somewhere".
Anyways, listen to the rock radio stations that are in or around Houston. If their stations are anything like a few stations here in St. Louis, they will keep you informed of anything going on that day regarding bands for upcoming concerts. When I saw them in Chicago a month ago, they kept mentioning how Alice Cooper was doing meet-and-greets, but unfortunately, no Iron Maiden.
This is probably the hardest of all, but could be the best possible chance to see them up close... try and find out where they are staying, or what time they are leaving/entering the venue (or hotel, or bar if they go to drink, etc). Because they have got to get to their vehicles somehow, which would open a 5-10 second window where they are in an open/public area (probably surrounded by police/bodygaurds). Again, you just have to keep your eyes/ears open, listen to radio stations, or just try your luck at the venue in meeting them.
I was so desperate to meet them in 2010, that I befriended a fanclub member on facebook just to ask her if she had any tips on what I could do to meet the band (she had SEVERAL pictures of her personally with all six members, so I figured it couldn't hurt to ask). She never even replied.... but that might be another piece to the puzzle: knowing the right person?
It is a dream of mine to meet the band, particular Bruce Dickinson. Just to shake their hand and say "Hello!", I wouldn't even need any autographs! I hope that I get to achieve it someday.