Riding in the heat can be a definite challenge, but it's still possible if you are smart about it. I didn't start riding until after I moved to Texas, but this is what I've learned about coping with the heat so far:

Timing:
Get out early! Getting out as the sun is rising will buy you a couple of hours riding time before the temperatures peak. One of my riding partners has kids, so in addition to beating the heat, he likes to get out early while his wife and kids are still asleep -- we tend to meet at first light and go from there.

Another option is to head out at the end of the day. I'm not as big a fan of this strategy, because the temps don't drop a lot until the middle of the night where I live, and the pavement still holds a lot of afternoon heat, but you do avoid exposure to the direct sun.

Hydration:
Make sure that you're well hydrated before you start, and take lots of liquid with you. On a 3 hour ride, I'll go through my two bottles and much more trying to alternate between water and a sports drink with the two bottles. My bf and I (as well as our local cycling club) will plan routes that allow for convenience store stops midway. This is a great chance to refill the bottles and grab some Gatorade or something similar.

Other tips/suggestions:
Finding routes that are shaded is always a good thing. Good amounts of shade can be difficult to find for road rides where I live, but the local mtb trails offer plenty of it!

I find that I handle the heat better if I am able to build my tolerance through the spring and early summer months, making sure to put in the miles as the weather is still nice but starting to heat up.

Know you limits. As much of a no-brainer as this sounds, it's easy to push yourself too hard in the heat. Spending the rest of the day trying to recover and rehydrate after a long hot ride just really isn't all that fun. If it's too hot, cut back on the pace or distance, or skip a day or two.

Having grown up in the northeastern US, I tend to equate Texas summers to Pennsylvania winters. (People spend a lot of effort minimizing their outdoor exposure as they rush from the house to car to office, etc.) The weather usually isn't so extreme that it will limit you from the things you want to do, but it does take a little extra preparation and forethought before you head out the door.

Good luck!
Emily