Do not hit angle shots. This just delays the match and defeats the purpose of the warm-up. Try not to spray the ball all over the place. Do not hit the ball as hard as you can or as hard as you may in the match. This is senseless.
Many players start competing in the warm-up.
This is not necessary and rude. Your inability to control the warm-up may also interrupt matches on adjacent courts. You can warm your body up by doing some mild jogging, walking fast or calisthenics. You can even do these things off the actual court. Do not expect to hit every ball on one bounce in the warm-up.