Why is it we can easily tell someone else when to put the brush down, but we can't recognize the moment in our own work?
Could making too many strokes be a sign that we are afraid of making too few? It can be tricky to tell the essential bits from the optional. Maybe we're trying to play it safe by putting in everything. That way we can be sure we've included the important part. Or maybe we're afraid the painting will be boring if we're too economical.
Either way, I think the first step is to deliberately commit the sin that we are avoiding. Make paintings with too little information on purpose. Risk leaving out something essential. If you do, I think you'll know it's missing. And if you don't miss it, you didn't need it.
Stay Abstract as long as possible.
Over-painting is directly connected to a tendency to hold on to information that is not really important. Just because you can see something doesn't mean it belongs in the picture. You can still use marks that are based on the abstract features of the narrative content of the scene to add complexity without adding information.
In the painting, above, Leslie Frontz emphasizes the barn and plays down the importance of the foreground and the sky. Look at those few strokes in the foreground. What are they, exactly? More than anything having to do with content, they are simply brush strokes, non-descriptive marks that keep the foreground from being vacant. In contrast, the strokes that comprise the barn refer to more specific entities; shadow, eave, siding, cupola.
I would certainly not call this an over-worked painting, yet there is no area that feels too simple. The artist recognizes that she can "populate" or activate an area by inserting a few marks that are not specific enough to be distracting. The context is sufficiently established so the viewer can supply meaning to those abstract passages.
For homework, find an image and make some decisions about which information you want to describe enough that it can be named, and which will remain abstract. Make some studies to verify your decisions.