As far as I know, you don't, other than deadheading - its a houseplant here, won't tolerate temps below 10-15 Deg C.

thanks, I do keep them indoors but I just wondered if cutting them back a little made them stronger.

Pruning always slows plants down, overall, but is sometimes necessary to cause branching, repair bad branching, or keep the size down. To make plants stronger, feed them (in moderation), give them enough sun, and water within their needs--not too much, not too little.