Know your airspace BEFORE you fly. So cal airspace is busy to say the least. Flying responsibly requires not only knowing where you are, but where the airspace is that we are not allowed into as stated in Part 103 (see below), and staying out of that airspace. Imagine you're an aircraft pilot on final approach into a remote airport out in the desert, low air traffic volume but still within its Class D airspace. You've contacted air traffic control and you're cleared to land. Suddenly, you spot a hang glider circling in lift half a mile in front of the runway centerline. HOLY CRAP. Now you've got to take immediate evasive action to avoid a collision when by all rights this airspace should be free of ALL other craft. Or just lifted off and some yahoo appears right in your flight path. Entering the various airspace classes mentioned in Part 103 without obtaining permission from the controlling agency is both illegal and irresponsible and endangers those using the airspace properly. Cross country flying is extremely rewarding and fun, but with it comes the responsibility of knowing the rules and playing by them.
Here's an excerpt from Part 103..."No person may operate an ultralight vehicle within Class A, Class B, Class C, or Class D airspace or within the lateral boundaries of the surface area of Class E airspace designated for an airport unless that person has prior authorization from the ATC facility having jurisdiction over that airspace."
For those who use Google Earth, the link below is an overlay that displays Classes B,C,D and restricted military airspace in 3d. Know before you go.