Playing as shaco means you are being a true killer. Without being noticed, and killing his opponent in a second is the basic guide for him. Shaco should neither focuses on damage nor ability power; he needs both of them (Although he should spend more money on damage). Shaco is a hard character, if you play well on asassin characters such as Akali, you should try playing with Shaco, the Demon Jester.

For me, I prefer to buy both damage and ability power items. Since shaco has a low health and armour, buying attack speed items does not help shaco because he is not a fighter but an assasin. Also, attck speed does not help shaco's skills at all (except his mirror image which dies fast).
Deceive - critical hit: higher damage the better, deals more damage when striking from the back
Jack in the box - higher ability power, the toy attacks with high damage (even though this thing seems to be useless, this actually dealts high damage; I have killed a lot of champions with this skill^^)
Two-Shiv Poison - flat damage + (1/2 of attack damage, 1 ap damage): the most significant skill for shaco's kills. This is almost the only way of dealing tramandous amount of damage to enemies.
Hallucinate - when the mirror image dies, it causes flat damage + 1 ap damage to surrounding units with magic damage. The explosion may shift a battle.

I prefer to not jungle with shaco. At the beggining of the game, shaco should buy Doran's Ring as fast as he can an set up his Jack in the Box as much as he can. Because shaco is a weak hp character in the beggining, it is a good idea to have your teammates protecting you from the opponent's gank. The blue buff is extremely significant to shaco because his maximum mana is not sufficient for him to use his skills frequently. Setting up Jack in the Box as a ward is recommended (not only this detect other heroes, but it also damages them a lot (feared also) which may help you kill the enemy easily). Try using ignite to improve your ap by 10(skill masteries) for better use of two-shiv poison (helps alot in the beginning).

Shaco should often get blue buff during the entire game. You may die a lot of times especially if the enemy champions have stuns, but you should use deceive for escaping. Shaco should have more kills than deaths for getting more stacks on Sowrds of the Occult, which improves his first and third skill, and killing minions, monsters and heroes.

With the items that I've suggested, shaco is one of the strongest chacter for 1vs1. With using all his skills, his item skill (Hextech Gunblade), and his summoner spells, he can use total 7 skills only concentrating on dealing damage to the opponent. It may be hard to control him with using all these skills. On the team fight, shaco should get involved a little later and aim for weak hp champions without being noticed by opponent's teammates. This is possible because he can use a lot of skills in a few second which shaco can assassinate certain champions before Shaco gets targeted (escape by decieve).

If you got 20 stacks on Swords of the Occult, your movement speed is improved and you can get things done in faster pace.