The same heavy demands placed on your engine during the summer months can also affect your car's transmission. Check your owner's manual to see if you're getting close to the recommended service interval for the transmission. If so, do it before you go.
You'll want to make sure your transmission fluid is nice and clean, since dirty transmission fluid is a good recipe for cooking your transmission until it reads "well done." If you're in doubt, we'd suggest you go for the service. Changing the transmission fluid before a long trip is cheap insurance. And, when you're getting it done, ask to have the transmission flushed — not just drained and refilled. That'll help assure that any gunk in the transmission is removed, so it won't cause any untoward problems when you're barreling towards the World's Largest Ball of Lint, in East Slackjaw.