This is a simple one. Between points, your only thoughts should be tactical. Don't think or worry about the score, about how important the match or the next point is, about nets and edges, or anything else except tactics. The very act of thinking about your next tactic should clear your mind of other thoughts, so it's a double-whammy. Keep the tactics simple, and once you decide on a tactic, clear your mind and let your training take over.
But tactics can have a broad definition and can overlap with sports psychology. If you are losing confidence, the tactical thing to do might be to tell yourself, "You can do it!" or the equivalent. If you are losing focus, the tactical thing to do might be to stare at something in the distance for a moment to clear your mind and regain focus. Everybody is different in this way. During my playing years I mostly attacked with my forehand, and so had to move a lot - so between points I was always telling myself, "Push yourself!", which was my way of saying "Move!" - which was my way maximizing my movement so I could, tactically, get my forehand into play.