Make sure every external door has sturdy, well-installed deadbolt locks. Sliding glass doors offer easy access if not properly secured. You can secure them by installing commercially available locks or putting a broomstick or wooden dowel in the inside track to jam the door. Never hide house keys under the door mat or a rock that is near your door.
Most burglars know that trick.
Instead give an extra key to a trusted neighbor. When you move to a new house or apartment, have a competent locksmith rekey the locks. All outside doors to your home should be metal or solid wood. Install a peephole in all entry doors so you can see who is outside without opening the door. Prune back shrubbery and trees so that they don't obscure windows or doors.