The Message: If you can't call yourself, have a reliable person call for police and an ambulance, if needed. Have the person say that you are an off-duty officer and give your description, as well as the suspects', so that responding officers know that you're the good guy. In a commercial robbery situation, it's safer for your family and friends to have a store manager or employee call, in case there are other robbers you don't see.
The Welcoming Committee: Try to have a responsible person, such as a store manager, meet the officers before the officers get to you. Instruct them to tell the officers your identity, your description, and your location.
You: Avoid meeting responding officers with your gun drawn, if you can safely do so. Holster it or put it in your pocket, but keep your hand on it. Look for the officers before they see you, so that you aren't startled when they arrive. Never turn on responding officers with a gun in your hand.
The responding officers are in charge, so do what they say and don't argue. If they tell you to drop your gun, drop it. As you do, turn it sideways so that it's more likely to fall flat instead of on the hammer or rear of the slide. Drop it where the suspect can't reach it. You can also step on it to make sure that the suspect can't grab it. If your gun is holstered, tell the officers, "My gun is holstered. I'm slowly raising my hands."
If you're involved in a serious off-duty incident, seek legal assistance early in the investigation process. Remember that the first duty of your agency's or municipality's lawyer is to your employer, not to you. Make sure that both you and your personal lawyer understand that while you need to cooperate with the investigation, you should not take pleas or accept settlements in court or accept disciplinary actions from your agency that aren't justified.