BUILD:
Boots of speed and 3 health pots -> boots of swiftness OR mercery's.
(Depends on the enemy team of they have stuns and so on, if not, then boots of swiftness for awsome chasing ability)

Phage + Zeal + Sheen -> Tritinity force
(I start with phage just because I can slow them down and some healthboost, then zeal for more crits and then sheen, sheen wont do anything because I dont have any freakin AP so thats why I take it after everything. And yes Trundle is made for This item!)

Bloodthirster
(Awsome lifesteal and increases your damage on rabid bite and normal hits. Cant say anything else about that... and by now you should already have won but if not:)

from now on can you start choosing between theese objects:

Madreds bloodrazor
(High hp enemies? NOT ANYMORE! they will more likly die in max 6 hits and you have like 1,8 attack speed)

Inf. Edge
(Crits... glorious CRITS. A great item for Trundle, And increases your Bite even more and now you're like a train that you cant stop... so far you get gang banged)

Frozen mallet
(Haha I haz ghost I can run! oh no wait, my feet is pretty cold and now Im like stuck in the floor and cant move while Im getting asspanked by trundles freakin club. No escape = quick death)

Manamune?
(Haven't tried it... But it should work)

An another bloodthirster
(More heal than fiddles freakin drain, and yes... Damage around 500-600 per crit and you crittin like all the time... I have about... 25% so far without inf. edge and high attackspeed and Im not done with the runes)

Force of nature or thornmail
(How can I describe this...? Oh hi Im trundle beat the shit out of me plz, oh no wait you cant because you guys takes about 0 damage! Meanwhile you beat the shit out of them instead)

Last Whisper
(Stacking armor? Not anymore!)

Skills:

Rabit bite
(You're not supposed to spam it. Use it right after one attack because Trundle will use it as soon as you hit the Q! It will be like 2 hits in one hit... It also increases your attack damage with 40 damage under 8 seconds and reduces the enemies damage with 20 for 8 seconds. Pretty useful to use on... Everyone!)

Contaminate
Ah, you cuse the ground and increases your movement speed with 40% and your attackspeed with 60%. Use this if you're about to flee, chase or if you know that the enemy will stay, use it and kill it within 2 seconds?)

Pillar of filth
(Ah one of my favourite stuff in the game. VERY USEFULL SKILL! Put it behind enemies in early games when they dont think that you're about to attack. Then run forward and bite off their head. They wont escape so far they have flash. or with mininum health you can also use this to save teammates by putting it between him and the enemy target, it will tear them apart and slow the enemy down meanwhile your friend runs to safty, you can also use it to check out bushes and so on for enemies)

Agony - ulti
(Decreases enemies Armor and Magic restist. And you steal 100(+) hp immediately)

How to put your skills:
In the start take rabid bite. (you will more likly take away at least 1-3 health bars if you do one auto attack and then bite and then one auto attack again.)

At lvl 2, take pillar you will need it if you havent got first blood yet and the enemies is a bit greedy put it behind an enemy that isnt aware of ya and then eat him up. Om nom nom (or to just tear your friend and enemy apart)

Lvl 3
Contaminate
I dont use it so often, It barly helps in the start, increases movement with 20% and some number on attackspeed. Just use it for escaping at start together pillar.

Lvl 4+ Lvl up bite as much as you can and if you cant, then use it on the pillar.

Lvl 6, 12, 18 Agony ofc.

When you got your bite and pillar on max, put everything on contaminate

Some proof:
http://img18.imageshack.us/i/efefdef.jpg/ Somehow it wont appear here...

(the 2 defeats = 2 idiot feeders on my team and op classes on the enemy team so then no can do)

Fighting:
Hit a pillar behind them, run in and use agony, ignite and hit once, bite their heads off, use the contaminate and hit them, now they should be dead.

Have fun

Added (2011-01-23, 9:06 Am)
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Ops one misstake about the sheen,
Get it in this order:
sheen
phage
zeal
-> trinity force