If you really want them filling in, by plants branching out, pinch off the tops when they are over 6" tall.

I pinch them off, as needed, largest ones at a time, and when they get to maybe 10", I slice about 2" off the tops at a time, and toss it around some areas of my garden, as an insect repellent. It also slows them down from flowering, and the flavor stays better, though eventually, they will flower - later in the season I'm spending too much time on the rest of the garden to fool with it!