Tip 1: Try to scope out as much of a room as possible before committing to a plan of action - some enemies can only be attacked by hiding behind certain bits of cover.

Tip 2: Keep an eye out for secret areas - vents are often hidden behind some foreground object, and often alternate paths through a room are available.

Tip 3: Don't deactivate a computer or blow a safe as soon as you've found it! Make sure the room is clear first, otherwise all nearby enemies will be on high alert.