I think that the best tip of advice for improving is to have a sound pre-shot routine. That includes sound fundamentals of stance, grip, bridge and most importantly visualizing the shot and cueball destination before you start. The two best drills I have used when time is short are: straight in stop shots, while working on the pre-shot routine on each shot; and the no rails drill....spread all fifteen balls out, no nearer than 8 inches from the rail, and pocket all balls without touching the rail with the cueball or object balls.