1. Motion Sensor Monitor. While these can be a bit expensive, they are certainly worth it. Much like home security systems, they need to be installed and activated, however some do not cost nearly as much as other systems. For example, after a quick web search I was able to find one monitor that was $199, the price of installation for most home security systems, and no monthly fee. This monitor can be placed over your door or other access point and can be controlled through your smart phone. This specific one also has a camera so that you can watch what is going on. If wanted you can turn it off or if you notice someone there that shouldn't be, a loud alarm can be issued from the device. If you ask me, that's a pretty good deal!
2. Strengthen Door Jams. If you have an older home, then you may be concerned about how much protection your locks will give you. If you head into your local hardware store, then the chances are that you can purchase and easily install something that will reinforce those door jams to give you some extra peace of mind when it comes to just how useful your locks are. Even if you can't find something you like, there are some heavy duty locks that you may be interested in installing that work much better than a standard deadbolt. If you have a rental property and cannot use either of these, you may be able to find a stick-on alarm system that emits a shrill sound if the door is opened.
While these two tips will cost you money, the sense of security that you will get from the purchase is priceless. Never cheap out on your own safety.