This is especially important when sniping. Use sound to your advantage by listening to movement sounds. This will often allow you to pinpoint the location of opponenents and avoid running into them headfirst. Finally, use headphones or loud speakers to make things easier to hear. Shoot through walls.
Some weapons can shoot through some walls.
Learn which weapons and which walls so that you can use this fact to your advantage. If you see or hear someone duck behind a fragile barrier, you can put them down with total impunity. Use hostages for cover. Opponents who kill hostages earn a cash penalty, so using hostages for cover is a good, if cruel, tactic. Reload before running out.