I definitely think they can be. While Willy does a horrible job at this, instead allowing them to plague his mind to the point he can no longer distinguish between them and reality, some people can use the events of the past to help drive them to future success. What about a student who recalls things studied in prior classes in order to do well in a current class or on a future exam? Then again, there's also the prisoner who clings to warm memories of home in order to endure their current lack of freedom, until they are released for time served.
I am also thinking about a soldier's experience, especially during deployment. They are away from home, unable to be with friends and family, and often in harm's way with legitimate threats to their well being and very life occuring nearly daily, if not more often. However, they can use their pleasant and fond memories of home, family, and loved ones to get them through even the most difficult of times, thus helping them to maintain their composure and keep their head straight in even the most difficult of situations.
So yes, I definitely think that memories can be used in this manner. If for no other reason, I remember that during my days in the Army when I was away from home, they helped me make it through.
In the play, Miller manages Willy's memories to reveal his past and explain his character. Through Willy's memories, we grow to understand what his life has been and why. For instance, it is Willy's memory of the woman in the hotel room and Biff's unexpected arrival that explains the shattered relationship between father and son. Willy does not manage his own memories, however. He simply becomes lost in them when he is under greatest emotional stress.
In real life, people can manage their own memories, to some extent. We can deliberately recall the memory of a time that gave us joy in order to relive it. We can deliberately recall a memory of some mistake we made in the past so that we don't repeat it. We can deliberately recall an event from the past in an effort to better understand what is happening in the present. Sometimes a memory will seem to come out of nowhere, uninvited, because something in the present, such as a sight or sound, has triggered it in some way. We can then choose to live in that memory for a little while or to push it back into our subconscious.
I would think, and I am no expert so this is just an educated guess, that anything that exists only in the mind can be manipulated. Interestingly, I think we do a lot of the manipulation ourselves. There are things we need to believe about our past, and I think our mind "manipulates" our recollections influenced by these needs. People tend to forget really horrible experiences and to remember the better times. But who knows if any of these recollections are accurate. I have a friend that I went to college with some 40 years ago. He often tells me stories about things that we either did together or which we were both at while they happened. I don't remember half the things that he tells me about; in many cases I don't have a recollection of even being there when they did. Whose memory isn't accurate? His? Mine? Both?
If you read Orwell's "1984" you will get a good exposition of the systematic manipulation of the past (memory) for political purposes.
There is an interesting recent example. In an survey of our best Presidents, Lincoln has recently displaced Washington as the best. I would like to know two things: how many of the people surveyed really had any knowledge of what these Presidents accomplished, and second (and perhaps more important) is the change in attitude toward Lincoln a result of all the press he's gotten from Obama? Wouldn't that be memory manipulation, even if inadvertent?