HOW TO... USE WEAPONS EFFECTIVELY
There are a variety of weapons at your disposal in each level, and choosing the right one for each situation can be crucial. Generally guns should be avoided as they are loud and immediately give you away if fired, though silenced weapons will get around this.
If you're going to use a gun keep it holstered until you need to use it, as being 'visibly armed' will also give you away as soon as anyone sees you. When firing guns, hold the trigger part way down to enter precision aim mode, making it much easier to line up that all-important headshot.
Away from guns, blunt objects such as bricks can be carried around without raising suspicion and used to bludgeon enemies. Sharp items like knives and screwdrivers can stab people at close range, or be flung over distance to get lethal throw kills.
HOW TO... GET UNDETECTED KILLS
The mark of a true Hitman is to make your assassinations look like accidents, so no one can be sure you were ever there. Always be on the lookout for locations and objects that can be used to cause accidents, which can be easier to identify if you find your target then follow them around to scope out their route.
If you're having trouble identifying these opportunities, by playing on either of the Enhanced difficulty settings and triggering Instinct when the prompt appears on screen you will receive hints to point you in the right direction. Of course, you can always take this advice forward into future playthroughs on the Professional difficulties.
If the kill you're going for involves tampering with anything, such as poisoning food or turning on gas, then make sure nobody is watching while you do it as this will expose you. Once you've got in and carried out your manipulation undetected, move away then simply sit back and watch events unfold.
HOW TO... SCORE THE MOST POINTS AND UNLOCK TECHNIQUES
The actions you carry out during a mission determine the score you receive at the end, and the harder the difficulty you select the more points you will earn. Achieving a high score on certain sections of levels unlocks new Hitman Techniques such as improved weapon handling and enhanced blending ability; these sections can be identified by accessing the Level Select option on the main menu then scrolling through the areas on each mission.
Knocking out or killing civilian and non-target characters gives you negative points and should be avoided when going for high scores, though this can be offset if you hide their bodies without being spotted. Recovering evidence from levels provides additional points, as does completing 47's Signature Kills using the environment, so you should try and factor these into your playthrough for improved scores.
The best way to achieve a high score for a mission is by obtaining a Silent Assassin rating. To unlock this rating, you mustn't get spotted or kill anyone except the target, however you can subdue other characters non-lethally and still attain Silent Assassin as long as you hide the bodies. Taking out your target with an undetected kill which looks like an accident is by far the easiest way to do this, though if you're in a secluded enough area you can get away with using a gun for a headshot.
For more help with achieving Silent Assassin ratings, our colleagues over at GamesRadar.com are putting together a Silent Assassin guide.