There are several powerful exercises that you can do using a mirror. You can use it to break bad habits or to help you visualize a bank/kick shot. For example, if your cue stick is not staying level, it is typically due to inadvertently raising or dropping your elbow. Fixing this bad habit usually requires more than having a coach just tell you that you are moving your elbow. And, a video recording only shows you what happened in the past. You need to know in the exact moment that you are moving your cue and associate your feel with this small muscle movement.
Using a mirror is your best solution because it shows you in real time what is happening. Set up a mirror alongside of you while you are doing your practice strokes, and watch your cue stick and elbow. Also note the spacing between your cue stick and the rail. I find that this exercise helps most players fix this in as little as only 1 or 2 sessions. First do it without using any balls and go from super slow strokes gradually to faster and longer strokes as you are able to keep your cue level. Next, line up object balls on foot-string of the table shoot them directly into the pockets without using a cue ball.
Once you are shooting these in with a level cue at different speeds and lengths of follow through, add a cue ball and shoot those object balls into the pockets while watching your arm in the mirror. Do all your practice strokes while looking back and forth at the cue ball and object ball then as you pause at the back of your stroke, look in the mirror as you deliver your stroke. During the pause is where the picture of your shot should be in your mind and your cue stick is aimed ready to hit your intended area of the cue ball. Stay still through the shot and also note how you finished the shot. Did you cue stick remain on the same plane all the way through the shot? Is your cue stick still in contact with your bridge hand, or did your elbow drop causing it to go up in the air. If so, start over with no ball and repeat this drill.
Another usage for mirror training is to be able to understand and visualize kick shots. If a ball is hit with no left or right English as medium speed it will rebound at virtually the exact same angle as it went into the rail. You may remember hearing in school, “The Angle of Incidence equals the Angle of Deflection”.