Masteries: I go 21/0/9. 21 in the offensive tree ofc ourse as any usual Shaco build, and where the points are placed adds decent beginning damage, more armor penetration, CDR, Spell penetration for your Shiv, and critical damage which works well with your Deceive. The 9 in Utility is for the improved experience, and to make your beginning blue last as long as you can while keeping the Offense 21 points.
Spells: I choose ignite and exhaust just because when you slow them with your Two-Shiv Poison passive, it sometimes isn't enough. So you use exhaust which allows you to land more hits. Of course, on a squishy person that wouldn't be needed, but one that's tanky would need more hits. Sometimes they could also get away with little health which is where ignite comes in to finish it. Also, ignite is very useful for things like healers. We all know Swain can be a pain, just as example, so you ignite him and he can't out heal your damage and you win.
Runes: It's really your own preference, I just like the Armor Penetration to hit harder especially early game. The seals are definitely your own choice. I just use what I have and it works for me; but tell me if you get somethings that are better. the Glyphs are for your MR because squishies die so easily to magic and every bit of MR helps. The quintessences are just HP ones because I feel they give me more confidence seeing as I have 100~ more hp at the beginning. I usually get around 750~ hp at start.

I start off with a Doran's Shield and an HP potion. I then go immediately to my blue buff (or if you have a jungler, the enemy team's) and start placing about 4-5 JitB's behind where the golem would spawn. When it spawns run up to it and hit it and the JitB's will do their thing and you usually wont even get hit by the golem before it's dead. Then just finish up the last two creeps and you get level two. Get your Shiv. Then go to your lane, and if there's an opening, try to get FB and if not, just harass the hell out of them with your Shiv. Once you get 1k gold, go back and buy Boots of Mobility and either head somewhere to gank if needed or you just want to, or go back to your lane and kill them there. Once you get 1337 (lol leet) gold go back and buy a brutalizer. Then next up after continuing your ganking/laning would be your Sheen. The Brutalizer adds nice armor penetration and damage for your abilities and the Sheen helps out your damage a lot as well. Your Deceive will hit a nice chunk out of a squishies health. Now you will continue the game as you are and save up 1850 gold for a B.F Sword. Then you will get the Cloak of Agility blah blah blah then Infinity edge. This together will make your Deceive so powerful. (ALWAYS make sure when you Deceive, hit the enemy form behind.) You will do an additional 80% damage from your sheen, extra 20% from your passive, bypass 30 armor (runes/brutalizer) and it will ALWAYS crit because that's what Deceive does, and do an extra 160% critical damage because of your Infinity Edge and your masteries.
All of this still leaves you pretty squishy, which is why I go for Frozen Mallet next. This will add more slow so No one should really get away from you as your hitting them hard. This also gives you a lot of health. Then the next thing you should do is finish your Trinity Force, just because it's upgraded from Sheen and will make you hurt more.:P Then just upgrade your brutalizer because it will add AS and MS basically for ***** and giggles. Then get what ever Item you want. I don't care usually the game ends around the time you get an Infinity Edge anyways. :D