DISCLAIMER: This guide was made in January of 2010. Items, champions, summoner spells, and the metagame have changed since then. Take this fact into account.
Jack(s): Jack in the Box
Shiv: Two-Shiv Poison
Shaco is an amazing physical DPSer due to his teleport/stealth + automatic crit, AoE fear trap, blind/slow poison that can be thrown as a nuke, and the ability to copy himself.
Shaco's main skill for ganking/killing and getting away. Deceive is vital to Shaco because of his squishiness. Be sure that if you're attacking with Deceive to be behind them for the automatic crit-hit.
Jack in the Box
Not that vital, but helpful in ganks, getting away, and a tiny bit of map control. Learn how to judge where to put these behind your target to make sure its fear effect lands as they're running.
Tiny slow and blind, but also a great single-target nuke. Try not to waste Two-Shiv just to deal random damage to enemies, just long-range kill-shots.
Hilarious skill, but also deadly. When Shaco gets his attack high, his clone deals massive damage as well and serves as a good escape goat or suicide bomb.
Shaco, like all melee-ranged people, is hard to farm with against ranged champions. To counter this crutch, try to have a tank or ranged person along with you, preferably someone with a stun and/or heal.
Farm it up. Then once you're lvl 6 and have Sheen, Greaves, and Vamp Scepter, go grab the lizard and start ganking around the map.
Keep up the map ganking, being especially sure to shut down enemy carries like Yi, Twitch, etc.
Long before now the 5v5 mid-lane showdown should have begun. Avoid being the person targeted and do NOT be one of those "we're probly gunna lose this fight so I'm gunna bail" Shacos. You are a DPS carry and you should be destroying enemies, even if it means you dying.
Jungle Shaco can either A: get the lizard then gank and then lane; B: get the golem (and lizard if wanted) then lane, or just straight out jungle.
Spam Jacks back away from the golem (they will glitch and not activate when the golem spawns otherwise). When the golem spawns, smite it to get it to come over to you. When the Jacks start to attack it, kill it, but be sure not to back too far away or it will go back and heal. Once the golem is done, get the spawn next to the golem for more cash. Buy Madred's Razor as soon as you can. Next move on to A: the lizard (if smite is ready) or B: the other spawn near the lizard (not the wraiths). Be sure to go gank after you have the liz and golem.
The jungle should be a piece of cake by now and you should be ganking everyone, everywhere while keeping the golem and lizard on and farming the neutrals.
Now jungling is just a side thing and you should be helping your team in laning.