The man looked a little worried when the doctor came in to administer his annual physical, so the first thing the doctor did was to ask whether anything was troubling him. 

"Well, to tell the truth, Doc, yes," answered the patient. 

"You see, I seem to be getting forgetful. No, it's actually worse than that. I'm never sure I can remember where I put the car, or whether I answered a letter, or where I'm going, or what it is I'm going to do once I get there -- if I get there. So, I really need your help. What can I do?" 

The doctor mused for a moment, then answered in his kindest tones, "Pay me in advance."