Tournament play should first and foremost be friendly and fun. In order to facilitate this, players have adopted common practices during gameplay.
Show up to the event
It is good practice to only sign up for events that you intend to actually attend, and to show up for the events on time. If you can't show up for an event you signed up for, it is good to notify the event organizers that you won't be there, or cross yourself off the signup list, so nobody waits for you unnecessarily.
This is especially important for events with a fixed list of available armies. Losing one player in a historical theme can break the event, and it deprives other players of the opportunity to fill that spot instead of you.
If you aren't sure whether you'll be at an event or not, make sure the organizers know, or don't sign up until you are sure.
This is just common courtesy if you've never played against an opponent before.
Describe Your Army
It is a good idea to always state the name and number of your army, and describe what each element in the army is. Point out distinguishing characteristics of your elements, so your opponent can tell where your general is, and can tell the other elements apart. This is especially important for elements such as Cavalry and Knights; Blade, Spear, and Warband; or Auxilia and Warband, that are based similarly.
State Your Intent
It is always a good idea to state what you intend to do, and to respect your opponent's intent as much as possible. DBA allows measuring any distance at any time, which makes it unnecessary to make precise measurements in some cases.
For example: if you intend to move just inside Zone of Control, then say "I'm moving just inside Zone of Control" (and place your element appropriately). Then, even if the elements are jiggled around a bit, both players know that you're within Zone of Control as long as no element has made another move.
It is also common to ask your opponent's intent, and to respect it. "Did you mean to move within my Zone of Control?" If they didn't have any specific intent, or if elements have been moved since intent was established, resort to measurement.
Watch the Clock
Everyone wants to take the time required to play well and avoid mistakes, but nobody wants to leave a game unfinished. The scoring system always encourages finishing a game rather than leaving it unfinished, so pay attention to how much time is left especially after event organizers start announcing that there's limited time left.