Your notice should be polite, friendly, and firm. Seek a meeting with upper management and try to iron out the problem in direct negotiations. Allow a month from the time they receive the notice for them to respond. If they haven't acknowledged your initial contact, call the president or CEO of the company and request a meeting. It is best to remain calm and allow your opponent to slowly come to terms with the fact that you are not going to tolerate theft of your intellectual property.
Companies have a personality, the same as individuals.
Some are basically honest, while others are as slippery as a used-car salesman. There is a tendency for mangement to attract other people with the same values or lack of values. No one likes to admit they are wrong. The same applies to businesses. Companies that intentionally infringe on patents usually have other problems with the way they apply technology.