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Pinch it pretty much the same way as any other plant, though it's usually not necessary. Just remember that tipping it will stop flowering for a few weeks while branches develop. The growing tip is usually so congested that cuticle shears are a big help.

If you are talking about an old, rangy plant that needs rejuvenation, cut about half of the longest stems back to shorter side branches (or the ground) feed with a 3-1-2 proportioned plant food, and try to give it more light. When new branches develop closer in, cut the remaining long branches back.