Her massive gun allows Tristana to fire on targets at a great distance. Utilize this to prevent your enemies from ever laying a hand on you.
Use Rocket Jump after you have stacked up your Explosive Charge on an enemy to finish them off with a burst of damage.
Use Rapid Fire to help stack up your Explosive Charge on enemy champions.
If you see Tristana activate Rapid Fire in a fight, stun her and try to back off until the spell dissipates.
Stand away from your creeps in a lane to take less collateral damage from Explosive Charge.