Crouching raises the chance of you not getting hit significantly.

Always look for available cover at all times.

The Q Spider can unlock secret areas and items by crawling through open spaces and scouting.

Never charge into a open space without an automatic gun and a lot of ammo.

Missiles are very easy to waste and can't be replenished so use them very sparingly.

Don't rely on hand-to-hand combat.

Always keep your P99 in reserve in case of emergencies.

You are a spy which means you need to be silent, crouch, walk, and silently disable men.