main things.

your main objective is the total map control. u need to learn hard the trap positioning, and you have to find the balance of your positioning depending on the situations. i mean, you have to take part in teamfights and also controling the key points of the map is not that easy as it said. later..

:::SUMM SPELLS:::
why not ingite? why exhaust?
teemo has enough damage, exhaust gives u survability, helps u and your mates chase down enemies. with the right timing combinated with your blinding shot can be really deadly to enemy melee chaps. makes them useless.
why flash? why not ghost?
flash is better than ghost for now. u already have a speedboost skill ("w"), witch dosen't have a huge cd. Escape works like: flash -> "w", and most of the situations its far enough to make yourself safe.
other options:
teleport is good, helps u defend and push lanes, but not mutch. clairvoyance makes your already good map controll even better, but thats not that important like exhaust and flash.
DON'T take cliense or fortify, those are not for you..

:::MASTERY TREE:::
obvious choises. take magic pen and CDr on offensive. u can also take defense instead of utility e.g. if you are not that good with squishy champs, dosen't feel yourself safe most of the game, but i say don't. utility is a key in this build. utility gives u mana regen wich you'll need because i'm takeing attack speed not mana regen on seals, gives you A LOT of CDr, movement speed.. utility is just awesome.

:::RUNES:::
M: obvious. magic pen, becaus you do damage with magic.
S: attack speed. i think i need a little explenation here. what attack speed does? why is it good. in the early game it helps you last hit minions a lot, because you have pretty low base damage. from lv2 toxic shot makes farming easier you say. well, maybie yes, but i don't know. the problem is probably with me, but sometimes toxic shot makes harder to last hit for me. sometimes i just can't last hit them. it just dosen't work.. that was the first reason why i pick attack speed. the secound is, attack speed works well with nashtor and berserkers. toxic shot has a magic damage impact damage, this is why attack speed is so effective on teemo.
G: ability power. u have good ratios on you skills, so flat ability power is very effective bouth early and late game.
Q: the same.

::ITEMS::
i don't want to expain that. my choice. where you can differ a lot from this build is the boots. mostly i take berserkers, but i like swiftness and mercury too. choose the best depending on the match setup.
THE ONLY BOOTS THATS USELESS FOR TEEMO IS THE NINJA TALBI. AVOID IT PLS!

there are some good items for teemo like malady, guinsoo's rageblade, mejai's soulstealer, zhonya's hourglass, madred's bloodrazor, sword of the divine (must have against jax). these are fine.

my item line is like.
doran's ring or boots +2hpp
than i complette my boots.
than i work on my nashtor, starting with fiendish codex all time! don't start with stinger, it just dosen't work.
if it's done, buy sheen, blasting wand, and complette it to litch bane.
here is a key point. if you pwn so hard you can decide what you need. more damage or survability. if you already have enough damage you should go for banshee (you dont want to give them 1000 gold right?). if you feel yourself totaly safe go for more damage and rock!
rabadon is awesome for your mushroom traps. if the enemy is about to takeing magic resistance, thake a secound to think about guinsoo.

finally i want to talk about your
::GAMEPLAY::

IMPORTANT---
your teamfighting is like: traping near rangeds, blind melee's and take down squishy targets first if you don't have madred's bloodrazor. mostly.

lane and farm as mutch as you can. if your team has a jungler, help with pulling the golem. these are basic stuff-- i don't want to talk about sutch things.
what most teemo players do wrong is mushroom positioning.

first of all, u can place your traps 2 ways. like a trap, and like a ward. what i mean, is if you place a mushroom to scout a place, don't place it where your enemies possible pass trough. e.g. if you want to scout a brush, place to the EDGE of the brust, not in the muddle of it. if you want to SCOUT the enemy golem, don't place it to the golem brush, place behind the golem. in mid and late game you'll have like 12-20 mushrooms on the map, don't tell me you can watch the mini map 24/7. mushroom disapearing time is 10 min, dont forget this.

u can stack up 3 mushrooms, if you are able, alwasy keep one for hot situations. don't reach 3 stacks, never reach it! that means u are not active, your cd is not cycleing. now you can probably feel why utility and cooldown reduction is impotant on this build.

early game.
key locaions are:
near towers: put a mushroom in front of you tower to avoid tower destroys. but don't put it too close, because ranged creeps wont go in it.
must scouted positions like baron, dragon, the river, golems, jungler ganking paths ect. ect.
mid and late game.
DON'T place traps near alone towers, all the good tanks will buy banshee and your traps can't defend towers buy themselfs. what you need is more scouting mushrooms and 1 stacked up mushroom for a hot situation.
important. BEFORE teamfights, always work out an escape path, and mushroom it. put traps near ranged carries and supports. enemy melee fighters gonna love them.

well, i hope everything is understandable. ask questions, comment rate and sutch. sorry for misstypes and bad english. maybie later i make some cleaning later.
ON DUTY!!!!!