Ignite
- Very good offensive pick, great for first blood, good for finishing runners (for example Tryndamere), and excellent against healers. I personally get this because I like strong early game presence, but it's rather useless for late-game as you already have Explosive Shot to counter healing. Still, if getting kills early on wins you the game, it's a solid choice. Ignite + Buster Shot will finish off everybody under ~30% HP.

Exhaust
- A very good spell, both offensive and defensive. After the change which allows it to reduce the damage of spellcasters it's practically a must-have for every single team. Combine it with the slow you already have to make it impossible for an enemy to get away even if they have flash. Also, the 10 reduced armor with Cripple make a significant difference in first blood scenarios.

Ghost
- Tristana is a very slow hero. Ghost counters this excellently, especially if you're playing a squishy build. Also, I've seemed to notice that the higher your Elo goes, the more people use ghost. It's practically a necessity to run Ghost + Anything right now, just so you won't fall behind your teammates and enemies.

Cleanse
- Great to break out of CC prematurely so you can Rocket Jump away. A real life-saver.

The first two are offensive, the second two are defensive. Mix and match them according to your playstyle. I personally roll with Ignite + Exhaust but any combination of the above are excellent picks.

Flash
- In my play way it's verry helpfull. We can dive turret jump over it and use flash accros the bushes to escape.

Bad choices

Fortify , Clairvoyance , Rally
- Leave these for your supporters.

Clarity
- You won't have mana problems with runes.

Revive , Smite
- Simply useless.