In advance of a heat wave consider adding several inches of straw mulch around your plants. This will help reduce evaporation from the soil and will regulate the soil temperature keeping plant roots cooler.
Make sure the plants are well-watered prior to the heat wave. Water as early as possible in the morning to reduce the amount of evaporation and prevent plants from drying out. Drip irrigation is ideal, but regardless of the method, avoid watering the leaves. Keep the source of water as close to the soil as possible. If you're using sprinklers, opt for the type that keep the water spray closer to the ground to minimize evaporation. (Cynthia Domenghini)