To begin with, plan your desert backpacks for the milder months. I was last in Arizona in late August while daytime temperatures hit 115 degrees F. Backpacking was not on my agenda.
When you do hit the trail, make sure that you carry enough water. A backpacker needs between half a gallon and two full gallons of water each day. Since a gallon of water weighs just over 8 pounds, there is a natural tendency to cut back. Remember, your pack will weigh less the further you go.
Plan you route to reach water supplies. Don't be fooled here. Talk to the local rangers to assess current conditions. I grew up camping on the East Coast. On my first backpack in Southern California in the summer time, I hiked toward a stream hoping to replenish my water supply the first night. Instead, I got a dry gulch with literally a single occasional drip oozing from the stream bed. Once you do reach a water supply, make sure that you filter your water before drinking it.
Be sure to have adequate sun protection. Get the best sun block you can find at SPF 45. Apply frequently. Manufacturers like Coolibar offer clothing that is SPF rated. Favor reflective colors like white and yellow over darker colors that will retain heat.
Carry enough food. Since you're burning more calories, you'll need more nutrition. When I backpack, I work through my fresh foods like sandwiches, fruits and hard cheese in the first day, then move into the dried foods like pasta, peas beans and freeze dried dinners as the trip progresses. For lunch, I'll carry jerky, pemmican or pouches of tuna fish. Between meals, I'll grab a handfull of trail mix.
Check local regulations about open fires. Some national parks don't allow them especially in the dry season. It's a good idea to carry a stove with more than enough matches for all your meals.
Be prepared for extremes in weather. It might be a pleasant 70 degrees during the day, but at night especially in winter, temperatures can drop precipitously. On my first night at the Grand Canon, I spread my sleeping bag out on a rock overlooking the rim, fell into a deep sleep and woke covered by a layer of snow. In summer, especially in Arizona monsoon like conditions can cause sudden thunderstorms.
Know the local flora and fauna, especially those that are dangerous. Rattlesnakes, like all reptiles have no mechanism for internal homeostasis. They regulate their body temperature through changing their environment. In the summer, they are more active at night. In the milder temperatures of the spring and fall, they can sunbathe or hunt during the day. If you see one or hear its heart stopping rattle, back off slowly.
Learn how to read a map and compass. A GPS is a handy adjunct, but no substitute for a good topographic map. Plan your route and stick to it. I like to build in a layover day, to just hang out, take local hikes or swim. If you are heading out alone tell the local rangers where you are going and for how long. If you break an ankle your third day out, this bit of foresight could save your life.
Carry a whistle and a mirror to signal if you do get stuck. Some of those spare matches you brought can also be used to mark your position with a smoky fire.
Finally, adhere to Clint Eastwood's adage as Dirty Harry, "A man's got to know his limitations." Backpacking is a great way to test yourself against the elements, but the desert is not the place to be foolish. Don't attempt trails or climb cliffs that are beyond your capability. If you need to rest along the way, do so without embarrassment.
