Cannabis plants have the capacity to regenerate after harvesting, going back into vegetative mode and then flowering again for a second, and sometimes third, crop. Your cannabis plant has put a lot of energy and resources into creating an efficient root network and all of those nutrient pathways are still there after harvesting. Seems like a shame to waste them J.
So how do you do it?
Although it is possible to just cut your plant down and get regeneration from the stump, it is more efficient to follow the steps outlined here.
When you first harvest your plant, remove the top 1/3 from the main stem. Remove the buds carefully from the middle 1/3 of the plant, trying to leave as many leaves in place as possible. On the lowest 1/3 of the plant, remove the tip buds but leave some of the nuggets on each branch. It is from these flowering points that regenerative growth will occur, so the more you leave the faster the plant will regenerate.
Now feed the plant with a high nitrogen feed. It is important to point out here that you will have fed the plant some nitrogen throughout its flowering phase. If it gets starved of nitrogen during this period it will not have enough reserves to begin regeneration.
Switch the lights back up to 20/24 (or whatever your preferred veg cycle is). Look for new growth within a week or two, starting from the lower branches where you left the budding points. As soon as you have a good bit of vegetative growth going, simply switch back to flowering mode by changing the nutrient and light regime as normal. You can expect a second harvest in as little as 2 months, depending on strain and conditions.
You can also use this method to get a crop from a mother plant by simply reversing the normal process. Crop your plant as described above, and then use the second generation vegetative growth to provide clones instead of reflowering.