For my items, I know they follow the general "Riot Recommended Build" for Varus, but I have tried this build and almost always get in the double digits for my kills. 29/18/8 was my last score. Also scores like 26/2/1 and 15/13/8.

+ High Attack Speed.
+ His black cleaver cuts through the tanks easy.
+ Wits end and Ionic Spark deal bonus magic damage, as does his basic attack with his blighted quiver.
+ I have even gone 3 vs 1 and killed 2 before flashing out.
Cons
- You need to farm hard to rush bloodthirsters.

As you work together with your team, even if you are less than a quarter of your health you can still kill an enemy champ chasing you if you are smart.
Varus is excellent at backing up a tank. Firing your hail of arrows, letting loose a piercing arrow. You can even go 2v1 once you get your phantom dancer.

The spells I use are flash and ignite. If you dont feel like using flash, exhaust is another option.
Flash is good because Varus is squishy and, like ashe, doesn't have a good escape from ganks other than just killing them.
Ignite helps take them down. Once they are low, ignite and shoot your Piercing Arrow and watch the health drop.

I would suggest just going head on with the creeps and shooting your Hail of Arrows over them. After that, just basic attack them, always trying to last hit so you get your speed up.

For team fights, I would recommend using Chain of Corruption, then hail of arrows so they have to walk across the slow to escape. Ignite them and basic attack till they are low enough, then let loose your Piercing Arrow.