Her true steroid is Ricochet. Because of her range, you generally have to swing at the tank, which is fine. Because you have Ricochet. Ricochet doesn't apply on-hits. It doesn't crit, apply slows, or respond to life steal. Ricochet grows one of 3 ways: +damage, +attack speed, and armor penetration.
Because of this, the most awesomest items to get are Brutalizer, Stark's Fervor, and Youmuu's Ghostblade. For late game you'll probably want a Bloodthirster (possibly replacing the Brutalizer) and a Guardian Angel which every carry should have at some point in the game. Also have Elixirs. Elixirs are awesome and Sivir can easily farm them.
Running around dealing damage as though your opponents all have 90 less armor than they do... That's going to work.
Also, with your ultimate and Rally... Well that's just going to hurt a lot. Even if you get chased down and have to get out of the battle (which you can easily do with E and R) your team still has improved Rally and the buff from your ultimate.