You should read the datasheet:
http://www51.honeywell.com/aero/common/documents/myaerospacecatalog-documents/Missiles-Munitions/HMC6343.pdf
Particularly the part about user calibration:
User Hard-Iron Calibration
The HMC6343 provides a user calibration routine with the 0x71 command permitting entry into the calibration mode and the 0x7E command to exit the calibration mode.
After entering the calibration mode, rotate the device reasonably steady for 360 degrees about the Y (Left - Right) axis and then 360 degrees about Z (Up - Down) axis. During the first rotation, maintain the Y axis at Level as much as possible. Maintain the Z axis upright as much as possible during the second rotation and until the exit calibration command is issued. The first rotation can also be done by rotating 360 degrees about X (Fore -Aft) axis. Then exit calibration.
The calibration routine collects these readings to correct for hard-iron distortions of the magnetic field. These hard-iron effects are due to magnetized materials nearby the HMC6343 part that in a fixed position with respect to the end user platform. An example would be the magnetized chassis or engine block of a vehicle in which the compass is mounted onto. Upon exiting the calibration mode, the resulting magnetometer offsets are updated.