My dog knows how to sit, but how can I get him to stay there?

How to teach Sit - not Sit, Sit, SIT, SIT!

Your dog already knows how to sit and does so all the time by himself! Your job is to get your dog to sit, by use of either a verbal or a hand signal, whenever you want him to do so.
 
A reliable sit (first time, and remain sitting till released or told to do something else) is the first thing that you should teach your dog to do. Why?

Teaching sit with modern reward-based training is equally effective and, if done correctly, the dog, learns the behaviour quickly, understands it and remembers it for all time.
 
 The up-to-date method for teaching an instant sit consists of the following steps:
 
  1. Get the sit by a) either waiting for the dog to sit of its own accord or b) by luring it into position with a small piece of food. Move your hand slowly upwards from the dog's nose to the back of its head the eyes and towards the tail. The head will go upwards and, inevitably, the bottom will go downwards into a sit.
  1. Mark the exact moment of contact (bottom hitting floor) with a 'That's right' signal such as an excited 'Yesss!', a click with your fingers - or by using a clicker.
  1. After the mark, immediately reward the behaviour (bottom hitting floor) with the piece of food.
  1. Practice the sit hand action at least 10 times a day for three days WITHOUT using any words. Your hand action, without food in the hand, soon becomes the hand signal for sit.
  1. Start phasing out the use of food. Only reward every second sit, then every 3rd sit and so on until the reward comes as an unexpected surprise on a random basis.
  1.  Reward from the other hand, not the one initially used to hold the lure. Start keeping the food somewhere else, not on your body, as on a plate or dish.
  1. On the fourth day, start saying 'sit' quietly AS the dog is going into the sit position.
  1. Once the dog definitely associates 'Sit' with the action of sitting then, and ONLY then, start introducing the word beforehand. It will become a meaningful instruction (sit), not a shouted and frequently repeated and irrelevant command (Sit, SIT, SIT!)
  1. Practice sits in many different places, inside and outside the house. Sometimes use hand signals only, sometimes use hand and verbal signals, and finally the hardest of all - verbal signals alone.
  1. Increase the length of the sits from 2 seconds to 3 minutes by small daily increments of 3-5 seconds
  1. Gradually start introducing distractions, such as when there is another dog 100 metres away.