I agree with Maestro's basic stategy, with a small variation in it depending on market conditions.
If a product goes deep green for more than two months and the factory utilization chart shows a large overcapacity I drop the price to -5/-5.
The GM corps will always sell their products at a -10% no matter what, so using -5% for a stategy will give a good chance of selling yours in a bad market.
Or you can use your CEO or country to direct buy at, say, +5% and hold it in your strategic reserve until the market changes.
As Maestro says, there's as many different stategies as there are people. His seem to be pretty good.