There are two methods that I am familiar with; Light-Poisoning, and Gibberellic Acid Treatment, both forcing female plants to produce male flowers and pollinate themselves. I have employed both methods, and both have yielded satisfactory results.
NOTE: YOU MUST START WITH FEMALE SEEDS/CLONES TO ENSURE THAT THERE WILL BE NO MALE CHROMOSOMES PRESENT.
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!
GIBBERELLIC ACID TREATMENT:
Select your favorite female plant and spray it from approx. two feet away (first under the leaves, then on top). This must be done 2 weeks before the plant is put into the flowering light cycle, thus the need to start with female seeds/clones. DO NOT SMOKE BUD TREATED WITH GIBBERELLIC ACID! Spray the plant again after 2 weeks have passed, and place it under 12/12 lighting. This plant will "hermie" and pollinate itself and other females present. It will not produce as much pollen as a pure male, thus less seeds. However, these seeds will be 100% female.
CONCLUSION:
Now, femenized seeds have also been known to produce hermaphrodites. This is just an evolutionary safety precaution to ensure the survival of the species in the event of environmental catastrophe. All seeds have the potential to hermie. Variables such as pH levels, lighting scenarios, fertilizer problems, etc. will also be factors in the outcome of the plant's sex. Just keep 'em healthy, and give them your tender, loving care, and you should be fine.