Do NOT get manamune, you absolutely do not need mana regeneration on trist. At worst, get it through runes.
Personally I run:
Armor Pen Marks
Crit Chance Seals
Attack Speed Glyphs
Armor Pen Quints
Yes, theres almost no mana regen there. You really don't need it. Just don't spam your skills early game, focus on last hitting unless you can get a first blood.
BF Sword > Finish Boots and IE
This build focuses on the only four stats that Tristana should care about: AD, crit, armor pen, and to a lesser extent, health. The emphasis being on armor pen.
This build absolutely requires a solid tank in front of you, and is in no way meant for lulzy 1v1's against Yi in the jungle. What it does is magically 5 shot carries and hits tanks hard enough to keep them from consistently being in range, finding a good spot to initiate. You want to focus squishie's that get into your range any time you can, but Trist hits hard enough after last whisper that you can take chunks out of whoever is in range. Make tanks pay for poking out.
The biggest hurdle in playing with Tristana is positioning. You can't just sit and fire off your cannon in the middle of a team fight. You have to use rocket jump to keep yourself on the edge of fights, just in range. Unless the team fight is almost over and you're trying to chase stragglers, NEVER rocket jump into the middle of a fight. I ALWAYS take both ghost and flash, or ghost and cleanse.
You absolutely have to have mid or a solo lane, and need to land as many last hits as possible. I commonly end long games as Tristana with 350+ minion kills.
Tristana being useless early game is a common misconception. I quite often get first bloods midlane by waiting for the enemy laner to overextend, rocket jumping on top of them, immediately flashing behind them, ulting, and then auto attacking them to death. Trist is a fairly decent ganker too, although its risky, because you generally have to rely on jumping on top of people early.
Basically, tristana needs to be played very cautiously. You absolutely need to take advantage of her range, and her ability to reposition herself. You need to stay behind a meatshield. If you start to get focused, jump out, but dont run off. Start potshotting from max range again.