Well you can try writing an idea down before thinking of painting or drawing it.
Imagining what you'd want to paint (and how you want to paint it) can come with a built-in limitation, so to speak, because you might be relying on what you saw previously, and what you can paint realistically, instead of simply thinking about the concept itself. Writing it down in words and staying true to what you wanted to do usually gets you past a certain barrier, then all you have to do is execute the description to the best of your ability on a canvas. It's also how any commissioned piece would go, because the client will just send you a text description of what he wants, the rest is for the artist to figure out.
Anyway, hope this makes sense.