Sivir is an extremely underplayed and underestimated champ. As a result, competent Sivir players often wreck enemy teams, as they have little experience playing against her. After playing her for a while, you'll, hopefully, find Sivir to be a very fun champ to use!

PROS
Insane pusher
One of the highest damaging, non-ulti abilities in game
Potentially able to damage entire enemy team at once with Ricochet
Can block a multitude of enemy abilities with spell shield, including Karthus ulti!
Ulti great for escaping, chasing or helping a team mate
Ulti now gives aura to any nearby allies at any time, not only allies who were nearby at activation
Now has a "Vayne-esque" passive

CONS
Quite squishy early game
Often mana starved
Will be focused in team fights due to damage potential
Boomerang blade deals less damage after passing through subsequent targets
Ricochet not as strong as it used to be

Armour penetration marks - pretty self explanatory. Taken on nearly all ranged AD carries. Significantly boost your Boomerang's early game damage.

Mana regeneration seals - ease off the mana starvation and allow you to stay in lane longer.

Magic resistance glyphs - help when laning against casters to negate some of their damage. Especially great if you're laning against an Annie, Ryze, LeBlanc, etc.

Movement speed quints - I take this on nearly everyone. Extremely useful for chasing down that fast Master Yi or escaping that annoying Singed.

Although, with the new masteries, I have been tending to stray from the traditional 21 points in one tree and 9 in another, I feel that this is the optimal set up for Sivir.

In the Utility tree, I take reduced death time (always handy) and reduced recall time, since none of the other level one options are really useful. I then take increased movement speed (woo, I'm even faster!) and one point in buffs for increased time with red, blue, baron, etc.

In the Offensive tree, I improve my exhaust and ghost and increase my attack damage. I then proceed to increase my attack speed and damage to towers, both of which boost Sivir's great pushing. I fill out the tree with only one point in critical chance ( you'll have enough with your PD and Atma's), points in lifesteal for early game sustain, extra armour penetration to compliment your marks, and the new Executioner mastery, helping you take down those fleeing, lowlife enemies quicker.

I like to get Boomerang Blade at level 1 for great harassing. Make sure you don't spam it too much, as it uses up a fair chunk of your mana. I then take a level in Spell Shield at level 2 and leave that until last. At level 3, I invest in Ricochet. I get On The Hunt at every available level, max Boomerang Blade first (by level 9), Ricochet second (by level 13) and Spell Shield last (by level 18).

Belong is a summary of the dos and donts of picking your Summoner Spells.

Optimal Spells:
Exhaust - a great spell to pick, especially when the other team has 2 or more AD heroes. Good for taking that Ashe out of the fight or taking on someone by yourself.

Ghost - another great option. Ghost, coupled with your ulti, allows you to escape from or chase down anyone.

Other Great Choices:
Flash - also good for chasing down or escaping, however, I feel Ghost is more useful after Flash's range nerf.

Promote - makes Sivir's already insane pushing absolutely ludicrous .

Ignite - take this if you spot a pesky healer, e.g. Swain, Mundo, Soraka, on the other team.

The Rest?
Your other team mates would be better suited to pick them, e.g. support should take CV. However, if you want to take a spell not mentioned above, feel free to. This is merely a guide, not a set of strict, must-be-followed rules.

I take Boots and three health pots at level 1, to help me dodge skill shots easier and generally out manoeuvre my opponents. Before recalling, I ensure that I have enough gold to buy Berserker's Greaves. The next time I recall I purchase a Vamp Stick and a B.F.Sword (which obviously stands for Best Friend Sword). Next, I buy a Zeal and leave it, and upgrade into a Bloodthirster. I then buy Warmog's for some survivability and an Atma's Impaler. Your attack damage is now over 200! I round off the build by turning my Zeal into a Phantom Dancer and buying a Banshee's Veil, giving me two different Spell Shields! However, note that if you activate your Spell Shield while your Banshee's Veil is active any enemy spell that hits you will break bother shields at once. Remember to use your spell shield after your Banshee's gets broken.

This build will give you a great amount of health with good attack damage, as well as providing you with a decent amount of armour and magic resistance.

Just some handy tips to improve your Sivir game:

Between attacking and using abilities on minions, thread in autoattacks against enemy champs to keep your move speed up

Try not to hit enemies with your Boomerang Blade when minions are shielding them, as the damage diminishes as the blade passes through units.

If you're low on mana, activate your spell shield and try to bait an enemy into using an ability on you. Spell Shield returns a handy 150 mana.

Be careful of enemies baiting you into wasting your Spell Shield - if possible, activate the shield when the enemy ability is in mid air, e.g. an Urgot Acid Hunter flying towards your face.

As I play Sivir more and more, and gain more and more experience with her, I'll add more to this guide. Until then, I hope that I have helped you to improve your Sivir gameplay, or, if you don't play her, that I have convinced you to try her. However, no matter what you do, the most important thing to do is to have fun!

- SpinTactician