1. Be Prepared
Know where you're going and what is required to transport your horse to that destination. This includes necessary paperwork to travel across country. Research online or call the location and find out what is required. Then speak with your vet about getting the necessary paperwork in order before you leave. Note: some requirements are time sensitive. Be sure to consider those timing sensitivities in your preparations. If you don't have your paperwork in order, it could mean extra time being stuck at border crossings for your horse in a hot trailer.
2. Directions and Maps
Have the correct directions to your destination. Look up the appropriate route on-line and buy a road map for the area to keep in your vehicle in case you need to detour while in route due to construction or weather issues. Confirm any overnight stays before you leave. Not being prepared could mean extra hours in a hot trailer for your horse.
3. Go Early
Avoid traveling during the hottest part of the day. The hottest part of the day varies depending on what part of the country/world you are in. Do your research before hand and know everything you can about your journey and your final destination. Most professional transporters get up early and are on the road during the cooler parts of the day, and allow the horses to unload and rest during the hottest parts of the day.