Keeping plants short?

I have a 5 ft. grow space.
I need to keep the plants at about 3 ft. So I can safely use my 250w MH.
How can I keep my plants on the shorter side? If I really have to I can get up to 3 more ft. Will this be neccesary?

u can top your plants or .. grow some lowryders or u can try a LST grow method...indicas pref... they dont grow as tall as sativas do

O ya dude I know all that. Unfortunatly I don't have a clue what kind of strain it is. What does LST stand for by the way? I might try that.