For me, the specific exercise is learning construction. I'm beg so take this with a grain of salt. Maybe I'm just forever bad at maintaining a mental image, but I'll just think of something, and try to describe to myself what I'm trying to draw.
For example, I'll say to myself, it's a sitting pose, and loosely decide on a body type, and then start constructing that until I get close enough to that mental image I had.
Don't try to just project an image from your brain if you can't, deconstruct it by thinking of the attributes of your mental image and reconstruct that onto the paper.