Hello MOBAFIRE users
this is my first guide.
The reason I like Sivir a lot is that it's flexibility is quite awsome, as it is a great hero for defense, a even better hero for tower overtaking and also an incredible support hero for team fights, soo we can do almost everything we want with Sivir.
Already updated for the changes of the update (02/01/2011).

Go for Exhaust and Ghost, Sivir is a Hit n Run champ, don't play with fire if you dont want to get BURNED!

I chose these runes for two reasons: they go very nice with the itens suggested and they are somehow good runes for a bunch of other champions, soo you don't need to loose your life playing this game to get some good runes and get bored of your champ just to start it all over again...
The dodge runes are a must, with the progress Sivir makes during the match, it will certainly be focus of mass traps, soo it will be very nice to bail out of 5 furious opponents that just want to murder you all the time because you took out all their towers when they were busy trying to take just one of yours down.
The coldown runes are essential for your ultimate, just because it's probably your enemys' worst nightmare.
The mark runes can be critical chance or atack damage per level, or even health ones if your computer isn't doing it's job latelly.

For the skills there is three ways you can take:

1 - for dumb opponents, take just lvl 1 for Spell Shield at lvl 2 of your champ, and use it to get back some mana from the not too bright enemy that keep shooting you with those "Here it comes again.." skills (you get 125 mana when you block an enemy spell with it, at the cost of 75 mana);

2 - for even dumber opponents, that ignore you even when you are at your fourth solo tower taken, don't even bother with the Spell Shield, go directly for the Ricochet skill and grab those itens as soon as possible;

3 - for the clever opponents, who know what dangerous is Sivir at the enemy team, you get that Spell Shield lvl 5 as soon as Boomerang Blade is lvl 5 (that means Boomerang Blade lvl 5 first) soo you wont die too much and get called a noob for feeding the other team...

No big secrets on the itens part. The 2000 mana bonus from Manamune is now not possible anymore, making it less usefull from it's past version, so not an interesting iten since the max damage one can get from it is around 20 + 50 (2% of 2500 mana). The Chalice of Harmony is a nice early game iten for Sivir, since Magic Resist and Mana Regen are two of this hero's weak points. By the end of the game, when you have already filled the iten spaces on your inventory, sell Chalice of Harmony because it's job will be already done, and go for a better iten. The last two itens, Bloodthirster and Trinity Force I chose, are really a game to game decision: if you are getting too much attention from the enemy team,go for Frozen Heart or Guardian Angel, if the opponents are too dumb to realize this far that the only reason they will loose this game it's because of you, go for Bloodthirster, Phantom Dancer or the Trinity Force itself, to keep the murder rate high. The other possible iten is a Warmog's Armor, Frozen Mallet or Rylai's Crystal Scepter, to get you that extra life you need to finish off towers within the blink of an eye.

Pros: your are the most handfull champ ever made, congratz.
Defense
Offense
Support
7 sec Last 2 Towers Breaking (with creeps of course!)

Cons: because you are soo awsome, you will attract some unwanted attention.
Focus
Focus
Focus
Run for your Life!

This Sivir build is all about team work, and by that I mean leaving your team to the mid lane while you obliterate the side lanes by yourself, or going back to defend tower in trouble, because people use to forget about the defense side of this awsome game.
For the team battles, Sivir is the ally Meeles dream and the enemys' worst nightmare (as I sad before), dealing damage at an unbelievable yet possible rate, soo get used for some Triple Kills and them some.

It's Sivir, farming is your middle name since you do it even if you don't want to.

Ok, this is simple and yet efficient, soo keep close attention and remember my words:

* don't take the middle lane at the first 5 levels of your champ, you will be no good help there solo unless you pick a really dumb enemy to feed your with your Boomerangs, I say that because the middle is a 3 way choise for your enemy to flee your Boomerangs, while the side lanes have only 2 options and usually 2 targets, making your life easier.

* don't be a hero, you are strong but you can't be targeted every time you go help your team or solo a tower, that would make your game really boring experience.

* defense, first defense, them offense, if you don't go back for those side lane towers of yours, no one will, and you will loose the game.

Well foks, hope you like it, please help me with suggestions, I'm allways up to learn new things in life. Thanks for your contributions for MOBAFIRE, because it's my main guide for League of Legends, and thanks for your time for reading my humble guide. Till next time.