Aside from giving them as much light as possible, when they get into Week 3 of flowering, start giving them additional P and K... these two nutrients will put more power into bud formation and density... I use General Hydroponics KoolBloom liquid with an NPK of 0-10-10 until the last two weeks before harvest when I switch to the dry formula of it - that has an NPK of 2-45-28...
Good luck!