Don't draw attention to yourself by acting suspiciously, e.g. hanging around near your target. In most levels, talking to people is the only way to progress as they'll give you vital items. If you can, always use your silenced weapon or fibre wire to kill. Killing is not always the best option. You can achieve many objectives without spilling blood.
Civilian deaths mean expensive clean up operations.
Too many kills can make you bankrupt. Think like a hitman - use your head before you use your gun.