how to keep stray dogs away from your backyard?

Get a cat ... and you won´t see any dog on your yard for your whole live ... trust me, I have a cat (male, 2 years).

If you know who the owners are, go have a talk with them. Many people think letting their dogs roam the neighborhood is OK. It isn't. Anything from getting shot, struck by a car to lapping up some antifreeze could happen. If they continue to let their dogs roam or you don't know the owner, I'd see if I could detain the dog somehow and call animal control. When they have to pay to get their dog back maybe they'll figure out that letting it run loose isn't so smart.