1) Crouching raises the chance of you not getting hit significantly.
2) Always look for available cover at all times.
3) The Q Spider can unlock secret areas and items by crawling through open spaces and scouting.
4) Never charge into a open space without an automatic gun and a lot of ammo.
5) Missiles are very easy to waste and can't be replenished so use them very sparingly.
6) Don't rely on hand-to-hand combat.
7) Always keep your P99 in reserve in case of emergencies.
8) You are a spy which means you need to be silent, crouch, walk, and silently disable men.