Always destroy boxes/crates, they will often contain ammo or health, especially in the long levels like Eye of the Storm or Nijmegen Bridge.
Never ever miss the opportunities created by a sniper rifle, you can zoom to incredible lengths with great accuracy.
When you start gathering explosives like grenades, use them to lure enemies around corners, they will always bounce off walls at U-Turn corners, so use them when needed.
If you don't need health, don't take it you will eventually need it and you will therefore know were to look, and you understand yourself that using a Field Surgeons Pack to heal you 10% back to health is wasteful.
To speed up blowing up a grenade, (e.g. if an enemy is about to kick it away,) simply shoot it, it usually works fine.
When you are not in a conflict zone, always reload your weapons, yes weapons - cycle through all you weapons and reload.
