Getting a good square sit such as Russel is demonstrating above is really nice to see. It's an easy thing to teach your dog at home. Essentially, find a food that the dogs likes. Extend the food to the dog enough for a sniff, then move the food just out of reach and towards the back of the head. Most dogs just naturally sit while trying to follow your hand. Say "Sit" as you are doing this and when the dog sits, then give soft praise and the treat. It may take a few repetitions to get the dog to actually sit but if you keep moving your body in front of the dog's, eventually the dog will sit.
Do this exercise several times. Once the dog does a good sit, begin to withhold the food and do verbal rewards. If the dog breaks the sit, then re-do the exercise. Remember to give a release word, like "okay" when the work is done.
For conformation, we don't want the dog to sit but to stand at our side and be examined by the judge. Sitting in the show ring is considered not desirable as the judge is trying to see the structure of the entire dog. I do use "bait" to get the dog to stand freely and happily. Some are food hounds and want to jump about until they realize that they won't get a treat unless they stand. Others decide that they would rather just be deadheads and even ignore the bait. All are just being Labradors!