Know your ability level: The trips on this site are meant for accomplished adventurers only! Sadly, even the easy trips on this site are beyond the ability of most people. Know your own ability level before setting out on any adventure, and gain backcountry experience on easy trips before trying anything that pushes your limits.
Use alternate navigation: Remember never to navigate by GPS alone. There are far too many inherent inaccuracies in consumer GPS technology, as well as the potential for equipment failure. Make sure you know how to navigate by compass and map also, and at least get some topos off of TopoZone before setting out on your trip, or buy USGS quads. I always plot my basic course on a printed paper topo before setting out, in case my GPS dies. That, along with carrying a good compass, serves as a backup to inherently fallible GPS technology. I navigated the backcountry for over 15 years before consumer GPS's became affordable, I recommend that everyone practice old fashioned navigation as a backup.
Bears: Bring your bear bell and bear spray, and if you don't have any, better get some. I live in the area and can tell you firsthand that black bears are everywhere out here, as are mountain lions and coyotes. Make sure you can recognize fresh bear scat and fresh claw marks on trees, and take the appropriate precautions or detours. Be familiar with prodecures for bear encounters.
Snakes: Rattlesnakes are prevalent in grassy foothills areas. If you are below 8,000 feet, be on the lookout for them. They have been particularly numerous in recent years. Be familiar with procedures for rattlesnake bites.
Lightning: Afternoon lightning can fry you where you stand, so plan accordingly. Most people start hiking early in the morning to be safe. If you are back at the trailhead by 1:30 pm, you are USUALLY (but not always) safe. Clouds develop quickly in the afternoon, oftentimes out of nowhere. Make sure you can read the weather, pay attention and recognize when storms are heading your way and head back to safety. The mountains will wait another day for you.