Topping your marijuana plants seems to be the new trend among advanced growers. The theory behind topping is to hopefully substantially increase the yield of the plant by creating more of the large and coveted "cola buds." Topping should always be done during the vegetative cycle, as soon as the clones are showing good root structure. This process should never be repeated more than twice per clone.
To top a plant, you are just basically cutting off the tallest shoot of your marijuana plant. The best place to make this cut is just below the last fully developed node. This will make the plant more bushy in appearance and allow the light to penetrate deeper into the plant, in turn developing more quality buds and a larger yield.
It is important to note that topping will add additional time to your grow, but the end results should make the extra week or two worth it. Also not all strains take too kindly to topping. Some indicas just take too long to grow, and may add up to an additional month or longer on your turnaround time. If time is of the essence, you will have to decide if the longer grow time is worth the extra yield...
Topping in action
This video is a good intro for how to top your plants. Runs a little long, but 5 min could increase your yield by 20% or more!
A perfectly topped plant. Take notice of the many cola buds!
Another perfectly topped plant
Untopped plants. Notice only the one cola bud running up the middle.