Yards are a good place to start, but they're only the tip of the iceberg. The possibilities are endless. I found four old silver dimes this spring on a boulevard in the street. Campgrounds in older areas are usually productive places to detect, especially if the ground has been used for recreation for many years. Almost every campground I've visited had a set of rules and if metal detecting is forbidden, it will most likely be clearly stated.
Federal land is another ticklish place to detect.
If it's a military post, the local Provost Marshall is the person to ask. Churches, older school yards, and ball parks are great places. I've found some of my best coins and nicest relics in them. Getting permission to hunt a church yard usually just means asking the pastor. He'll either give you an answer or turn you on to someone who will.