Use your team: You are part of a team of three. You can quite easily tell team mates where to go and to kill certain terrorists first. Use them to enter rooms via certain doors and to heal each other should they get injured.
Know your weapons: There are quite a wide range of weapons available in the game as well as equipment - it is worth playing around to see what weapons suit your style of play the best. Some of the various grenades can be very helpful for certain situations so think about what equipment you carry.
Use the map: Pressing the back button during missions brings up a map showing some key info such as the location of spotted terrorists and ladders, rappel points etc.
Enemies: Enemies tend to move around a bit differently each time you play so it can be hard to pin point each enemy in this walkthrough - they also react to noise etc. so if you are loud when you enter certain areas they may kill hostages or gather together to hunt you down. Use the map to locate some of the terrorists when you can.
Use the snake cam: A key piece of kit. Use it under most doors to check the way forward is clear. Also if you highlight and enemy with the cursor and press the back button it will order your team members to kill that enemy first on entry.
Blind Fire: In areas where you are under heavy fire it is worth using the blind fire to force enemies into cover to allow you to re-position or to get a better shot at them.
Re-load: Try keep your weapon fully loaded when you can - never know when you might face a room full of enemies. Also note that the enemy has to reload too - an ideal time to shoot at them.