Another tip is that you should not overdo it. Tell only a few "over the hill" jokes which will be really funny. If you tell too many jokes at a time, it will not only get boring but monotonous. Never ever make the person feel really old, especially if YOU are younger! The point of telling these 40th birthday jokes is to have fun and not to make the person feel worse.
Make sure that you hit your friend's or relative's funny bone with 40th birthday jokes, gifts or gags that will not only guarantee laughter and entertainment all around but make them feel better about their balding pate, expanding middle or thick glasses! You could also get a funny singing telegram to surprise the person at home or at the office. This is sure to send colleagues and co-workers into splits of laughter.