keep heat usage to a minimum. Heat can translate into split ends galore, particularly when used inappropriately. Try alternative methods (i.e., braidouts, twistouts, roller sets) to stretch your hair.
If you must use heat: Apply a good heat protectant from root to tip beforehand. Also, if using a flat iron, use a relatively low or moderate heat setting and only 1-2 passes. Lastly, use quality products and tools to minimize heat damage.
Heat protectant - Carol's Daughter Macadamia Heat Protection Serum
Flat iron - GHD
Blow dryer - Conair Infiniti (tension method)