How do I make smoother yogurt?

I've just made my own yogurt for the first time (using the instructions found here, though I also added powdered milk to thicken it: http://www.makeyourownyogurt.com/make-yogurt/what-you-need). I kept the milk at 185 degrees for 30 minutes to help thicken it, and I incubated for about 6 hours. Even though I whisked it well after incubation, it's still kind of grainy. What can I do to get it smoother like store-bought yogurt?