Most air leaks in the GC occur at one of three places, the nut on the end of the GC/MSD interface, the injection port, or the septum. Leaks at the nut on the end of the GC/MSD interface are covered in Finding an air leak in the MSD with the MSD leaks.
Leaks in split/splitless injection ports can sometimes be confirmed by switching the injection port purge on and off and checking the abundance of the ions associated with leaks (primarily m/z 28 and m/z 32). If the abundance of these ions with the purge off is much greater (x100) than the abundance of these ions with the purge on, this is confirmation of a leak in the injection port.
A septum will leak after too many injections. The septum should be replaced at least every 100 injections. Keeping a beaker of septa in an oven at 250 degrees C at all times will eliminate the need to condition a new septum once it is in place.
A Merlin Microseal(TM) is highly recommended over conventional septa. The microseal eliminates the need for septa and lasts for tens of thousands of injections without leaking. It is most appropriate for use with an autosampler. It only works with untapered, blunt-tip syringes.
The Agilent Technologies application note: Optimizing splitless injections on your GC for high performance MS analysis.