The key is to ride the market like a surfer catching a big wave, getting off before you crash into the beach.
Materials prices often rise and fall as people try to get into the market to earn a few credits and the inevitable under cutting begins. If you set the trend you will earn big credits and if you keep moving you will always be in front of the wave and not paddling to catch up.
For example, while looking for a particular widget you notice that there aren't very many of them available (or ideally, none available at all) and you decide to start making said widget and selling them on the GTN. Because you are currently the sole supplier, you start making mad bank.
Then the GTN vultures start to circle. They see you with that big fat kill and decide they want a piece. They start making the same widget. The smart ones sell at the price you've set, the dumb ones start the price dropping as they try to be the lowest price on the list.
You, being smart, stop making said widgets and instead start looking (or smarter, had already been looking) for your next score.
This strategy is simple and works for materials and crafted goods. It won't do you any good to post the 200th item of a particular type for sale, you've always got to be looking for the under served market.
Or, as most economists would call this, "Supply and Demand." Economics 101.
Very well said, though. Of course, 99% of most sellers have no clue what this means.