Always have a pen & paper with you. It's useful for remembering things during the mission and for writing down things that happend during the mission that might be for taking a look at again after everything. Keep your finger off the trigger. Keeping your "firing" finger rested on your middle mouse button, instead of the fire button. Helps to prevent an accidental and potentially fatal shot at the worst possible time. In the event that you need to alt-tab (switch focus away from Arma and to another program temporarily) for whatever reason, hit your Escape key or bring up your in-game map before doing so. When alt-tabbing back into Arma, a mouse click can be interpreted as a shot. Having your Esc menu up, or your map, will prevent this undesired behavior and remove the risk of a potentially fatal shot.
