Take more water than you think you'll need. Hauling a little more weight beats running out too soon.
Using a Camelback for water is highly recommended -- they hold a lot, make frequent sipping easy, and keep water much cooler longer than any bottle.
Use high SPF sunscreen and re-apply after a few hours.
Take a wide brim hat for off-bike breaks.
Colored synthetic fabrics provide much more sun protection than cotton, especially white cotton which equals about SPF5.
Take some high energy food I take Fig Newtons or peanut butter & bagels, others use energy bars.
Start riding about sunrise (which is early in the summer) to beat the hottest part of the day. I don't ride at night, except on rare roads with virtually no traffic.
Headsweats or other do-rags keep sweat from running into one's eyes.
Have the tools, parts, and knowledge to fix problems llikely to occur (flats, lost bolts, etc.).
Riding in deserts isn't something to be dreaded; most provide beautiful scenery and relative isolation. Just be prepared for the conditions