In case something happens to your original plans, be flexible enough to roll with the punches. You may encounter canceled flights, overbooked hotels, closed roads, or places you'd planned on visiting closed for the day. Have backup plans for alternate routes and activities. Bring at least one good travel guide with you that lists all the region's hotels, sites, etc.
When planning on staying in a region for more than a day, research other sites in the vicinity that you may want to visit. For example: if you're going to Palenque, make sure you set aside a day to take a tour to Bonampak and Yaxchilán. (Visiting Yaxchilán really makes you feel like an explorer. The only way to get there is by helicopter or a 45-minute small boat ride up the Usumacinta River. There are very few signs of civilization here; nothing to spoil the spirit of adventure.) If you still have time at Palenque (you should be there for at least 2-3 days), take a couple of hours to visit the local waterfalls at Misol Há and Agua Azul. (If you really feel like exploring, continue down the road to the Mayan ruins of Toniná.) If you're near Merida and have time to kill, visit the ruins at Dzibilchaltún. If you're going to Tulum (not by tour bus), take some extra hours to explore the ruins of Cobá. If you're going to Uxmal, save a few hours for the nearby series of Puuc Mayan sites: Labná, Sayil, Kabah, and Xlapak. And in big cities with airports that you may be passing through (like Guatemala City and Villahermosa) check to see if there is a museum of Archeology to go to. Tip: if you have several spare hours in Villahermosa, visit the La Venta Park. Here you can see numerous sculptures by the Maya's predecessors, the Olmecs. The statues and classic Olmec "big heads" are all displayed in a park-like setting.