Early in a match you should test your various serves against an unfamiliar player. Which serving motions work best? What proportion of the time should you serve long, short, or half-long (where second bounce is right at the end-line)? Should your deep serves be fast or slower, spinnier ones that break more? How often should you serve varying degrees of backspin, sidespin, topspin, or no-spin? Should you serve more to the backhand, middle, or forehand side? Should you serve from the wide backhand to get an angle into their backhand, from the forehand to get an angle into their forehand, or something in between? Which serves maximize your chances of following up with an attack, and which serves maximize the chances of the receiver missing outright or popping the ball up? Should you try out your trickiest serves right at the start, to see which ones work, so you can keep coming back to the ones that do, and so your opponent has to worry about that serve, making your other serves more effective?
When receiving, how often should you receive aggressively as opposed to control receives? Should you attack all the long serves or should you sometimes return them more passively? Against short serves, should you push long, short, or flip, and how often for each? Where should you receive to - wide backhand, wide forehand, or perhaps attacks to the middle?
Don't wait until the match is nearly over to figure these things out - that's way too late. Try to get a good idea of these things early on.