The roof

 

Look up for any holes in the roof. You never know who's there. Headshots aren't pleasant. If you wear a metal helmet, it'll be even more unpleasant. :)

Use red cloth

 

...to mark dead players.

Couple minutes before the photo was taken I got a series of BBs from another player. Since I looked exactly as on the photo, I must say that guy was blind...

 

MP5 with its long clip is not the best replica to be used while lying on the ground. With that long clip you have to keep your head high to be able to aim through the sight. The results are frequent headshots. Received, not scored!
You might turn the replica so the clip would be horizontally, but then the hop-up would cause the BBs to turn...