A good memory is dependent on good mental agility which is again dependent on mental exercise.
Regularly solving crosswords (or at least trying), playing chess, working out mathematical problems, putting together jigsaw puzzles and any other exercise that requires concentration and brain power will help keep your mind fit and healthy.
It's also important to continue learning even if you're well past retirement age and perhaps feel you have no reason to learn anything new. Studies have shown that those who are deprived of learning stimulation are more likely to develop Alzheimer's Disease or other dementia than those who continue gaining and using knowledge.
Hopefully, by using the above tips, you'll no longer find your banker's card being taken from you because you've keyed in the wrong PIN, your appointments won't clash, and you'll be able to greet everybody with their name when you meet them.