-Start with a short strain.
-Use a broad short pot.
-Keep vegetative growth time to a minimum. Run 18-6 (instead of 24-0) and once the plant starts to show sex, flip to 12-12.
-You might consider running LESS than 12-12 for flowering, too.
-Training: Bend tops, tie them down, and/or use a screen to keep the plant as low as possible.
-Make sure to keep the light near the plant to reduce stretching. In a PC box you might have an easier time RAISING the pot rather than lowering the light(s).