Not Updated For Current Season
How do you play shaco? Make sure that when you hit them with deceive you do SO much damage that you either 1-hit them or do enough that you will kill them very shortly afterwards. The early sheen will increase the damage you deal in ganks amazingly. Ideally you will gank whenever it is physically possible to do so. If it all goes well you should have all the items up to trinity force by 25mins (I gank enough that I generate 15-20 K+A by this time). Then when the team fight's start you should simply put a box down when needed. And then watch and wait until the time is right to 1-hit that carry/nuke.
The randuins and banshees are there because if a game drags on they will allow you to stay alive in team fights and actually contribute. Once you have trinity force however, your deceive crit from behind can hit for amounts around 1.5-2k depending on your target. Easily enough to finish off carries below the 60-70% health mark.
Your job then is to kill the carry, then act as an off-tank.