Teaching a Dog to Sit
To sit on command is not really a trick, it's a basic command that any dog should be trained to obey.
How to teach your dog to Sit
Stand next to your dog. Place one of your hands under the dogs neck, and the other on top of their hindquarters. As you say "Sit" gently push up with your hand under their neck, while pushing down with your other hand on their bum -- guiding them into the sitting position.
Most dogs don't need much force at all, and will sit with little pressure. As they sit, praise them. Repeat this a few times and they will soon learn to sit by verbal command only.
Now your dog knows how to sit :)
Tips for Stubborn Dogs
So your dog just doesn't seem to "get it"? One thing to remember with all tricks is NOT to repeat yourself. Don't say "Sit, Sit, Sit.....Siiiiiit...Sit!" Your dog is a lot like a child, repeating your commands is nagging, and your dog will learn to ignore you.
Say "Sit" in a commanding but calm tone once. Make them sit using the above method.
If you are not strong enough to get a big dog to sit:
Another tip is as you say sit, hold a small treat above their head, as their head goes up, their knees will bend a little and you will then be able to push them down into the sitting position.
You may also want to check out: Pet Supplies