Things never to tell the IRS

Never voluntarily tell the IRS anything, unless they specifically ask you for the information.
Never try to explain everything away with reasonable excuses or showing that there was good faith on your part and you never had any intent to get into trouble.
Only answer what they ask for and nothing else. Do not allow yourself to become too friendly and enter into a casual discussion with an IRS employee.
Taxpayers have the right say nothing to the IRS, and instead to have a knowledgeable advocate represent them any point during the discussions with the IRS. This can be a CPA, tax lawyer or other tax professional who is given IRS form 2848 Power of Attorney.

Never tell the IRS anything that is not true.
It can be a criminal offense if you get caught. Do not rely on a false belief that there is no way the IRS will know you are not being truthful, they are trained to ferret out this type of information.
If the taxpayer believes he may be criminally responsible for something, he should not speak directly with a CPA or most any other tax professional either. Rather, a lawyer with attorney-client privilege must be retained before saying or admitting to anything. If two IRS employees show up at your door, nine times out of ten, one of them is there to be a witness in any forthcoming criminal prosecution, so, immediately get yourself a tax lawyer and only then retain a CPA by going through your lawyer.  See Kovel letter and U.S. v. Kovel.

Never answer a question asked by the IRS, unless you thoroughly and completely understand the question and the potential consequences of your answer.
If you do not have a good grasp of the applicable IRS law and regulations regarding the topic being asked about, you may tell the IRS that you do not understand, or that you do not know, or that you do not recall and you would like the opportunity to temporarily stop the discussion until you retain a tax professional to advise you and represent you so that you are better able to provide a fully informed, truthful and complete answer to the questions.
The tax professional can be a CPA, EA, or tax lawyer who is given IRS form 2848 Power of Attorney.

Never agree to meet the IRS at your office.
Do not invite the IRS to begin a fishing expedition. Taxpayers have the right to meet at a mutually convenient location, this is best to me somewhere other than the taxpayer’s home, office or workplace.  Often this is best handled at the office or a CPA who is given IRS form 2848 Power of Attorney.

Never promise to pay by a certain date or file by a certain date, before you are positively certain that you can and will follow-through on that promise.
Failure to fulfill any agreement you previously made will be a black mark against you as it is notated in your file. With this in your file, the IRS may limit your ability to ask for leniency in the future. If, due to some circumstances you anticipate missing a scheduled payment or filing date, call the IRS to explain the reason for the delay and indicate when they can expect you to resume timely action.