A box filled with the same mail from day to day is a sign that the occupant is not home. If your apartment unit has a mail slot in the door, cover it so nobody can flip it up and see that the apartment is empty. Don't buzz anyone into the building whom you don't know. If you don't already have a wide-angle viewer, install one in your door. Even if you live on a high floor, don't think your unit is impossible to access from the outside.
Some thieves climb or jump from balcony to balcony.
When you're out, keep windows locked, and sliding doors locked and defended with security bars. If you're out for the evening, leave a light on, even if you're on a high floor. Professional thieves can pick out a completely dark apartment from the ground. If you're keeping any belongings in a storage room, don't rely exclusively on the room's lock for protection. Use a high-quality padlock on your own storage unit door.