You can top your plant which is effectively taking the main stem and taking it as a cutting. This will cause you lower branches to grow more vigorously and you may have to top some of them too. You ca also tie the tops of the branches down though I have no experience with this. I think it's pretty simple though, just do a search of the forums.
It also depends on the strain. I'm growing Master Kush and the strongest ones tripled in height from 12"(they're now 36").You may not need to worry about it... You plant may not have 6' in its genetics. I know a lot of people are always saying your plants can double to tripple when you put them in flowering, but that is b/c they are flowering when the plant is 8".
I would say most indoor strains top out somewhere right about there. I've veg'd the same strain until it was ~24" and it ended up going to ~36". Then, veg'd to ~18" and with some LST'n they got to ~40".It also depends on the strain. I'm growing Master Kush and the strongest ones tripled in height from 12"(they're now 36").
Interesting - thanks for posting - I'm guna check this out!easy, if you dont want to top, just add a flowering additive called phosphoload. not only does it stunt the growing height almost immediately, it promotes bushier plants and tighter buds. i think you onl use this the first 2 weeks of flowe.