The Handler and Dog ("Sit" command help)
First of all, "sit" is the most reliable and most frequently requested command by owners. Most owners hold a fair to medium "sit" command with their dog. It can be learned fast and should be taught early. If you rescue your dog as an adult it should be re-taught or cleaned up early in the relationship also. No matter what age - it is always positive when dog and owner work toward a common goal....the experience alone sets a precedent for the relationship, demonstrating to one another how and what works (and what doesn't).
No matter the dog, the breed, or the age - "targeting" the sit is easiest, and allows the handler to work in the command as they see fit. Whether that is with a verbal cue like "Sit", a hand command or gesture, or both. "Targeting" would simply consist of using a high quality treat and pinching it between your thumb and forefinger and allowing the dog smell it.... then raise the target hand (treat hand or what the dog will be eyeing) to a position maybe an inch or 2 above the dogs nose and give your cue. Thats it. Repeat until the dog tires of eyeing the treat at an elevated angle and sits down by default. Reward it with treat and tons of happiness and watch the behavior multiply on your cues. Can be accomplished off leash and should be something fun, almost a communication effort between the handler and owner. The more you demonstrate the ability to "eye" his behaviors and target them for him - the more he will respond in tune with you.