May 31st 2011
Generally speaking, the magnetic declination for Southern California is approximately 14 degrees east (as is frequently shown at the bottom of topo maps). That is, Magnetic North is 14 degrees east of True North.
However, the magnetic field of the Earth constantly changes due to the currents of the flowing magma beneath the surface. Therefore, your compass should be adjusted every two to three years.
The current magnetic declination correction information can be found on at least two sites on the Internet: NOAA and Canada. Frankly, the Canadian site is easier to use. The Canadian site is: http://gsc.nrcan.gc.ca/geomag/field/mdcalc_e.php