Gain increasing amounts of AD or AP, Adaptive over the course of the game.
As a heavy Crit stacking AD Carry, Fleet Footwork is a fantastic rune choice on Trist as it allows you to sufficiently sustain in lane and reposition with the burst of Movement Speed.
Overheal pairs nicely with Fleet Footwork and Taste of Blood, allowing you to take short trades and come out ahead thanks to the extra bit of health going in.
If you think you're going to have a hard time staying at full HP thanks to enemy poke, take Triumph instead.
Tristana is all about getting in as many auto attacks as quickly as she can. Early game this means a quicker [E] stack and late game is means more crits.
As an AD Carry Cut Down is the most reliable source of damage amp as most other champions in the game will have more health than you do.
Absolute Focus on Tristana will provide you with a ton of Attack Damage when you stay above 70% HP. This should be relatively easy to do with your long attack range and Life Steal from Fleet Footwork, Bloodline, and items.
Gathering Storm is your late game insurance policy, making you a 30min+ game monster.