Doors are the main entry points to your building and they need to be as secure as possible. One way to make sure they are completely secure is to use the best quality locks you possibly can. They should be new, strong and British Standard so that you know you can trust the quality. These should also be installed by a professional who knows exactly what they are doing.
You can add extra security features to your doors if you are looking for something extra. Think about night latches to make it harder for someone to gain entry. You could install bars that help to prevent forced entry and security chains that make it safer for you when you are opening the door to a stranger. You also need to make sure that the doors themselves are doing their job. If your door is getting a little bit worn out, consider investing in a new one.