Lighting and a dialed environment as @cemchris is saying. Stay away from nutrient line with PGRs in them. They will get you tighter buds, but they will slowly poison you too.
You'll also want to reduce your nitrogen intake in flower, cutting it out all together with a month or so to go. Excess nitrogen in that stage can creat looser buds and foxtails. Lots of tutorials on here, just take a look around and if you don't see something, let us know :)