Play the warm up as you could normally. The sensor will detect that you are warming up and it will not count this as part of the match.
Bounce the ball before you serve - this will help the sensor detect serves and get your winners and losers correct.
Just play your matches as normal - you don't even need your phone. Just go on court and play. You only need to use the app to sync your data once the match is over.
Don't play shots that aren't valid. It's especially tempting at the end of a rally - say the opponents ball was out of court, or it was a double bounce - to play "the shot you wanted to play". The sensor will probably read this as a shot that's part of the actual rally, even though you know it's not.
Try not to play shots in between rallies. It's fine to pick up and bounce the ball etc. But hitting a shot can lead the sensor to think you have started a new rally.
Try not to hit the wall hard with the racket head. Of course, this happens to everyone in the heat of a match, but the less you can do it the better.
Don't use the sensor as an extension to the racket handle! It's not there to be gripped and used to hold the racket... if you hit a shot while holding the racket with the sensor you are likely to break the dock that holds it on.
Don't use Live Mode for a practice session, then immediately play a match! Once you go into Live Mode it treats the remainder of that session as a practice session. If you want to do practices and matches together (and also if you want to play matches back to back) you must SYNC your data in between.