You can LST (Low Stress Training) or top the plant. Topping the plant will encourage the lower shoots to rise to an even canopy level, while LST-ing takes a bit more work because you have to tie shoots that are "too tall" down. You can also try to use fluorescents, as they put out little heat and therefore won't cause stretching. Also fluorescents don't put out as much light as HPS or MH bulbs, so the plant will inevitably be smaller. Lastly, you want to flower it early to keep it short, because sativas can up to triple in size during the flowering stage. Any combination of these things and you should have a low-ridin' sativa!
PS: NEVER top the plant when it's flowering... do this a week before flowering at the very latest, but I would suggest you do it when the plant is much younger. As for LST-ing you can lean over a flowering plant if you want to, just be careful because the stems are much more brittle/not flexible during flowering