Water some more. One of my pet peeves with gardeners is something I called social watering. This is where you stand there with the hose and spray your garden while you talk to your neighbors. You look righteous and your garden looks refreshed, but if you stick your finger down in the soil you'll notice that everything just below the surface is bone dry. Make sure you give roots a good soaking. It helps to rough the soil surface up beforehand with a hoe to break up the water-resistant crust, and then count to 10 as you water each plant.
Put on some sun screen! Even on cloudy days, the sun this time of year is especially intense. Apply it at least every two hours, even if it necessitates the use of a headband (think Ben Stiller in Dodgeball) to keep the sunscreen/sweat combo from dripping into your eyes. Ultraviolet light comes in several flavors, and I was surprised to learn that although the glass of your greenhouse or car window blocks the UVB that makes you tan or makes Vitamin D, it does not block the UVA that ages your skin or makes you look like crepe paper.
Plant everything. All the little pots and market packs, cuttings in jars, grocery bags with gift plants in the driveway and alleyway — none of them are getting any younger or healthier! Whenever there's a break in the heat, get out there and find a place for them. All will benefit from a drink of compost tea or worm-juice tea upon planting. I make worm-juice not by juicing worms, but by throwing a handful of worm compost into the watering can. This will clog up your nozzles, so remove them before doing this job.