[cantharis]

Many of us, especially if we are growing an expensive strain, would like to get our plants to make a small amount of seed for next year (or couple of years). But without turning our whole crop seedy.

Keep one male alive, but keep him small by keeping him in a small pot. Keep him well away from the girls - my girls grow on my roof terrace, I keep any male in a corner of my garden downstairs. Carry out this procedure about 4 weeks into flowering, when there are a good load of trichomes, but about 4 weeks before you expect to harvest, as it takes seeds that time to fully mature. Mature seeds are brown, immature seeds are white.
Take a plastic freezer bag, put it over the male and give it a shake. You will collect a good lot of pollen in the bag - you may not actually see anything, but it´s there all right. Now put the bag around one flowering branch of one female, secure it with an elastic band, give it a good shake, leave it for a week.

At harvest, that branch will have a hundred or more seeds, but the rest of your crop will remain sin semilla.

I am not a geneticist, but I have done this with White Widow, and have found the progeny of the seeds to be every bit as potent as the parents.