I have read one time that our brain needs mental jogging. So the way to retain a good memory is to continuously read, think and learn. One method I've learned regarding remembering where you put something is to constantly visualize in your mind where you left an object. Say a ballpen in your drawer, picture the ballpen being put in the drawer repetitively. I have learned that when we do things unconsciously, like putting down an object almost not attentive to what you've done, we easily forget that. Why? Because it did not register in our mind. So leave things on places the way it should be, make records of important things, that way when you exercise the mind by doing these constantly, it's when memory retains its function well.