Ha ha.

As someone who could well do with losing a stone or so, I liked this one -

Mr Smith was overweight and went to the doctor for help.  "What I want you to do" said the doctor "is eat normally for two days and then skip a day.   Do that for a fortnight and then come and see me again, and you should have lost about five pounds"
Two week later Mr Smith returned to the doctor, who weighed him and was amazed to find that he'd lost twenty pounds.  "Good heavens" said the doctor "and you did that just by following my instructions?"  "Yes" said Mr Smith "mind you those third days were tough.  I thought I was going to die!"  "What" said the doctor "from hunger?"  "No" said Mr Smith "from skipping!".