A study was performed to see if offering candy to patrons would help create goodwill and better tips for restaurant wait staff.

Test one placed the candy at the register or counter and this had virtually no effect on tips.

Test two had the wait staff place one piece of candy/mint for each patron with the bill/check at the end of dinner.
This test improved tips by around 3%.

Test three had the wait staff place two pieces of candy/mints for each patron with the bill/check at the end of dinner.
This test improved tips by around 14%.

Test two had the wait staff place one piece of candy/mint for each patron with the bill/check at the end of dinner.
When the wait staff started to walk away, they were instructed to turn back around and give each patron another piece of candy and a smile as an after thought.
This test improved tips by around 24%.

This is the rule of reciprocity and it does work. The only hitch is that it could backfire if not used selectively. If other patrons see you using this approach too often, they will think it is a ploy or scam.

A better way to get more tip money is to repeat verbatum each order you receive so it is clear to your client that you were listening and did indeed get the order right. 

People like people who are like themselves and repeating the order exactly as it is given offers the perception that you are like the person placing the order.

The best sales people mimic their prospect. example: The prospect folds their hands - the salesperson folds their hands.

It makes you more like your prospect and they actually do not notice the activity, but they like it.