There are some interesting invention strategies which don't appear on the usual lists of control and loosening heuristics. As Diane Ackerman notes in an essay entitled "O Muse, You Do Make Things Difficult," Dame Edith Sitwell used to lie down in an open coffin. D.H. Lawrence climbed naked into mulberry trees. And the poet Schiller kept rotten apples beneath his desk lid: whenever he got blocked, he would raise the lid and inhale deeply.
Incidentally, Ackerman notes, Yale researchers have proven that the fumes of rotten apples do work
Whichever heuristic you use, here are specific professorial expectations related to invention. Not all, of course, apply to every type of writing. But they represent a good general set of directives.
When you've finished your draft, run a Geiger counter over it to see what's hot. Then push this material.
The Indian sage Krishnamurti suggested that the function of education is to free your mind from all tradition so that you can discover the truth as it is. Received truth, he proposes, is limited and limiting.
Be self critical. Criticize the ideas and attitudes presented, but turn the same level of criticism on your own ideas and attitudes. A mature mind seeks its own criticism.