Step 1: Choose a command. Make sure to choose something that won't be easily confused with outside stimuli. I use two firm, but gentle taps at the base of the tail.
Step 2: Using a treat, lure the dog into the sitting position by allowing him to nibble on it, and bringing it up above his head.
Step 3: When his bum hits the floor, reward.
Remember to keep these sessions short - even just a few minutes at a time, to keep things interesting for the dog. Always end sessions on a high note, and set your dog up to succeed. If you find yourself, or your dog getting frustrated, take a break and try again later.
While these methods have worked for us, it is important to remember that every dog is different and what works for one, may not work for another.