Rule is simple: Don't draw too much.
Let me explain. Mind likes to imagine things from clues. When it understands the clues and gets the story the clues tell, it is happy. We are aiming for that happiness from our audience. If you give too many clues, then it is like a puzzle with too many conditions. If you are not talented, those visual clues you provide may not square with the puzzle/story. So the trick is to give as less clues as possible to solve the puzzle.
So let us see what all we can avoid. First thing we can avoid is the outline. Rather just fill in the details you think interesting. For example, when you draw a tree, don't draw the trunk branch and outline of the leaves. But start from the part that interest you most. May be it is the leaves. Take some time and draw them here and there. Now if you think the picture is incomplete just add some lines for the branches. Don't draw both sides of the branch. Only part of the branches and trunk are enough. But never miss to hint on the interesting details. May be it is a hole in the trunk.
Next thing we need to add is some depth - a play of light and shade. It is quite easy than you think. Put some cross hatching in the corners were you think will be relatively dark. If it is total darkness, then put the hatching on the center of the object itself. Again, don't over do it. Just a hint; and do it with confidence.