I use Fastrack liquid dispersible. Although the container is expensive, (about $44 shipped http://www.marvalstables.com/happybellies/what_does_it_cost.htm )
one canister should last you many months, I estimate about 6, with one dog.
Testimonial== had ROTTWEILER (ie massively more air and smell produced lol) who was so bad we would not let her even sleep in the HALLWAY. When we had a sick pony we put on the horse formula, I decided to try my dogs on the liquid dispersible. The gas stopped. Okay I know that it is very healthy, new studies on HUMANS show probiotics may have some really impressive results beyond just healthy digestion. But I won't lie. We kept it in stock to prevent Rottweiler gas.
Yogurt sounds good, but the daily cost comes to far more than Fastrack even if you get them on sale at 75 cents or less.