Empty the yogurt onto a large cheesecotton cloth. Gather the edges and hang it or place in a collander over the sink. Leave for 6 to 8 hours to drain the whey into the sink. Squeeze the cheesecloth to drain whey faster.
Add it back into container, using a spoon to scrape the thickened yogurt from side of cheesecloth.
You get about 3 pounds of homemade greek yogurt with live and active probiotic cultures for a fraction of the cost. (Plain yogurt from an Indian store - low fat and full fat options available - costs $3.50 - $6 for a 5-pound container.)
Rinse cheesecloth before adding it to the washing machine.