This is another question that I get asked all the time. The answer to this questions depends on the state you live in. Many states you have a "Duty to Inform". This means that you are required to inform the officer that you are carrying a firearm first thing.
In Mass, you are only required to inform the officer if you are asked. But you should keep in mind that it is always better to let the officer know yourself then to let him see it on his own.
If you are directly asked you keep both of your hands on the steering wheel and say
"Yes officer. I have a LTC and am currently carrying on my "right hip" (or where ever you are actually carrying the gun) how would you like me to proceed?"
Then just follow his commands. There is no reason to be nervous, you have done nothing wrong. The officer may ask you to step out of the vehicle so that he can secure your firearm for the remainder of the traffic stop for his own safety, or he may say thank you for letting me know please just keep the firearm holstered and don't make any sudden movements towards it. Remember the officer just wants to make it home at the end of the day too!
If you decide to inform the officers without being directly asked either because you live in a Duty To Inform state, or you would feel more comfortable letting them know or you are concerned that the officer may see it when you go the grab your wallet. Then just follow pretty much the same procedure as if you were asked. Place both your hands on the steering wheel and calmly say
"Sir or Mam I just want to let you know that I have a Valid LTC and am currently carrying on my (fill in the blank) how would you like me to proceed."
Again just follow the officers instructions and everything will go smoothly.
One thing to pay special attention to is if you keep you wallet in your right back pocket, and you have just informed the officer that you are carrying on your right hip, (as most people who carry do) and then you quickly go to reach for your wallet in the back right pocket you may cause the officer in that split second to think you may be reaching for your gun. I suggest that at that point if your wallet is on the same side as your concealed carry firearm that you ask the officer directly if it is ok for you to reach on the same side and grab your wallet.
I would say something like this " Sir my wallet is in my right rear pocket near my firearm is it ok if I grab my wallet out now so that I can provide you with my drivers license?"
Then just listen to what he asks you to do and do it.
Everyone needs to understand that traffic stops are one of the most dangerous things that a police officer does, and they are way more likely to be killed during a traffic stop then any of the other calls they go on. They know this as they are taught this in their Police Academy, and because of this they are in a heightened state of awareness. They don't know if you are just coming home from the grocery store, or a wanted violent felon who just stole the car of a guy who was at the grocery store, and is looking for the first opportunity to hurt or kill the officer and escape.
If you are calm, polite, respectful and LISTEN to what the officer tells you to do everything will be fine, and who knows it might just get you off with a warning!