Keep your head on a swivel
Cant tell you how many times this has saved me. Dayz is all about knowledge. Knowing about another player before they know about you is key.
But even if you dont notice them first, its best to notice them at all before it's too late.
I was just leaving Stary Sabor after finding the last wheel I needed. Just left the safety of buildings and was starting up a hill east into an open field towards a line of trees I was going to follow. I was keeping my head on a swivel like I always do even though I hadn't seen any sign of players in Stary. There he was a couple hundred meters behind me running right at me, geared. He was coming right for me but without a great line of sight and I would be out if view soon on the uphill.
The key thing here was I shouldn't have seen him and he was charging at me oblivious that I knew he was coming for me. I barely made it to a bush, dropped my backpack, and turned with my DMR ready.
He came into view, and I think confused, as he was expecting me to be in the open, (easy pickings) heading for the treeline (I mean where else was I going?) and that would have been that if I hadn't noticed him before. I hit him with 2 .308 rounds and he dropped. I started for his body still keeping my head on a swivel only to notice a slight movement at the base of a tree in the direction of the body.
I zoomed in, it was his duo prone aimed at me, I hit him with .308 too and ended up with their KAM, M4A1, DMR, and a ton of grenades.
Keep your heads on a swivel gents, and hit your shots.