What techniques can one utilize to increase the vividness of one's mental imagery?

My mental imagery/imagination/mind’s eye is quite vague and unformed. Some people I know claim to have very vivid mental imagery, and I would like to, if possible, train myself to do this more effectively.

I have, on occasion (usually just before falling asleep), had experiences of extremely vivid mental imagery for very brief periods of time. I was also able to control these to some degree, in essence “sketching” with my mind. I want to be able to do this in a waking state for longer and controlled periods of time.

I also posted a related question asking how vivid the typical person’s mental imagery is.