Pressing jump then crouch is an important technique to master. It'll allow you to jump onto taller obstacles, and will let you jump through narrow windows or gaps. Practice it and learn how to use it well. Always be unpredictable. Don't do the same thing over and over.
Opponents learn fast, and good opponents learn really fast.
If you score an easy kill or ambush one time, don't do it again right away. Take alternate routes through the level ocassionally. Being predictable will quickly get you killed. Always travel in groups. The odds of survival are greatly increased when you have backup.