Wai, like many other gestures e.g. no loud or bang when talking or shutting doors, reflects your overall etiquette that is perceived to link to your family background and stand in the society.
Thais are rooted from the hierarchical order of society; our wai thus has different height levels. To wai beautifully, it takes time to practice (and many details that i do not even remember!).
Not sure if a proper wai is at all significant in today modern lifestyle. But since i was put in the Queen school, hope that I can be little helpful here.
We wai when we want to (1) pray, (2) greet, (3) thank you, (4) apologize, and (5) denote a receiving / wai back.
Wai Monk = Thumbs between eyebrows. Index fingers touch forehead.
Wai Parent = Thumbs touch nose (parents are your breath of life). Index fingers between eyebrows.
Wai Teacher / Master = Thumbs touch lip (teacher words of mouth make you a better person). Index fingers touch nose.
Wai Senior Person than You = Thumbs touch chin. Index fingers touch lip.
Wai Same Age / Younger Person than You / or Wai Back = Thumbs touch between breast. Index fingers touch chin.
Note, bend your head down slightly a bit to cater the reaching of your index fingers.
One of the the best and simplest explanations of the Thai traditional wai that I’ve come across.
With thanks to ConcreteAngel of ThailandFreinds.com