I like tristana, but my main issue with her is that her ult is rather weak. But maybe I just haven't found enough creative uses for it, as I don't play her much. One fun trick though, is as soon as you hit 6 and they get near your tower, rocket jump over their head and ult them into it
Doesn't work on good players though, as they smell that trick a mile away. You should also try her with phage/frost mallet. Its one of her key items.
Oh, and as far as runes, I don't think its been said, maybe its obvious maybe not, but you get better returns for certain types stats on certain runes over others. Like blue is caster stats, where you get best CD reduc, ability, and mana regen. So picking 3 stats is the best bang for your buck. Red is physical DPS, IIRC, and yellow is defensive.