Wandering around aimlessly is going to get you killed. Instead, ensure you’re a lithe and nimble killing machine by employing these movement-based tactics:
Be Like a Shark
Never stop moving. If you’re stationary, or even remaining in a specific area for a long amount of time, you’re easy prey. Employ crouching and cover where appropriate, but only when under attack. Never appear in the same place twice, and your foe should never be able to learn your routes.
Control, Not Camping
If you find yourself in an area with cover (especially in Zone Control maps), it’s worth learning where enemies are going to appear from, and train your (long-range) weapons at this spot. But don’t simply camp there; constantly move so you can more easily react to a foe appearing in an area you didn’t expect, as well as to remain mobile so you aren’t hit yourself.
Hip-Firing Strafe
As you should be constantly moving while in combat (unless you want to die quickly), you need to learn how to effectively fire and move simultaneously. This is achieved (without the slowing penalty of sight-aiming) by attempting a circle-strafe—sidestepping in one direction while turning your weapon’s aim in the opposite direction. Why? So you can more accurately utilize the inertia of your movement while manually aiming.
Getting High
If a particular area or map has multiple vertical levels, attempt to reach and lurk around the top floor, if there are good and open views of areas below you. It’s always easier to aim at, and fire, on foes below you, especially as you can retreat and use the edge of the floor as cover, or drop down and finish the job.