LIGHT-POISONING METHOD:
During the first three weeks of flowering, turn the lights on for an hour during the middle of the dark period. That is, 12 hrs. on, 5.5 hrs. off, 1 hr. on, 5.5 hrs. off, and repeat for the first 3 weeks, after which you may return to the normal 12/12 light cycle. This causes a plant to go "hermie" and pollinate itself, as well as any other female in the room. You must use plants originating only from female seeds or clones to ensure that no male chromosomes are present. The resulting seeds will produce NO MALE PLANTS!

Source for above http://www.overgrow.com/growfaq/1638

Good Kannabis Growing!