How you water makes as much difference as how much you water. Set sprinklers for early morning hours to maximize the water absorbed by your greenery. Water for longer periods of time. Avoid frequent short watering, it encourages plants to keep shallow roots.
A good layer of mulch insulates soil, blocking sun and reducing water loss.
Avoid using fertilizer as it will push the plant to grow, adding stress.
Just as humans wither quickly when sick, plants do the same. Confronting any pests or fungal diseases will give them the best chance to survive the heat.
For plants that are sensitive to heat or withering, a shade cloth is key. The cloth is made to only let a certain percent of sunlight through to the plants below.