This cannot be emphasized enough. Know whether they work during the day, whether they have children (and, if so, what they look like), and what their needs are. Do you live next to an elderly couple that stays home, but sometimes needs help? Or is there a teenage driver in the family? Are there small children you need to look out for? Knowing these things helps you be aware of who is in your neighborhood and what could be suspicious.
Have a way to reach your neighbors if they’re gone and something is amiss in the neighborhood. Feel free to contact them if you see any out-of-ordinary or suspicious behavior around their home or if you’re gone and would like them to check up on your house. You might even help develop a neighborhood calling tree.