You must always remember that "there's always one more." After you secure a kill, always check your surroundings before looting as there may be sneaky players waiting to catch you off guard.
The "Always One More" rule is holds true no matter how long you have been playing the game and we tend to forget it too easily at times after looking at precious loot.
Furthermore, you should also clear areas before you loot POIs. Always check your corners for campers.
Walk, Don't Run. If you are out of combat, running will draw attention to you and you do not want that. If you are not fighting, slow down and walk from one place to another.
Stop and Listen. Whenever you can, stop all movement and try to listen for hostiles who might be sneaking up on you. With no map in-game, audio is your best friend.
Don't Be Inactive. Always do something, to keep blood flowing into your hands. "Cold" hands can cause your reaction time to go down and lose 1v1s. Keep checking your surroundings and keep repositioning into better areas.