This is my idea for Tristana's item build, it's put in order of what I plan to buy:

Boots of Swiftness
Frozen Heart
Infinity Edge
Warden's Mail
Phantom Dancer
Phantom Dancer

The second Phantom Dancer is NOT a typo error. I'm serious.

By the time I get all this, I'll have:

A decent amount of speed
Slowdown effect with my attacks
Big Damage due to high critical rate & high attack speed
Good armour

The skill build will go like this:

By level 3, I'll learn all 3 basic skills to be prepared for most situations.
From there on out, I'll max Rapid Fire first, followed by Explosive Shot then Rocket Jump.

I find that Tristana is definitely not a runner, but in order to rack kills, I was thinking that she should get close with Rocket Jump first, then activate Rapid Fire in order to do more damage per second. Frozen Mallet should ensure that the opponent will stay in Tristana's firing range for a longer period of time. Rapid Fire will also aid in the farming process. Buster shot in my opinion should be used as a defensive skill to shake off gankers. Explosive Shot can help to harass opponents in a lane and keep them away from the towers.

By late game, the 2 Phantom Dancers will max out my attack speed for sure, but the real function is the amout of critical strike it gives. This will furthur increase the damage per second done to an enemy. The attack speed is a sort of bonus.

To be honest, the Phantom Dancers were slapped together at the last minute. I could'nt find another item with a higher critical rate.