Hang out on your porch or in your front yard. It’s hard to get to know your neighbors if you don’t ever see them. Just by being out in front of your house you can give off a welcoming vibe that encourages interaction. If I am out in front, especially with the kiddos, every neighbor who walks by, even those I don’t know who may live many blocks away, still smiles and says hello.
Notice that infrastructure matters. A neighborhood where most or all of the houses have front porches will support community better than one with all garages in front. A pretty yard that's open to the sidewalk is more welcoming than a tall fence or hedges. These features rely a lot on the quality of citizens -- in bad neighborhoods, the houses look like fortresses.