Hi everyone im new to mobafire while I like reading guide here I wanted to try and make
a guide myself. Since I mostly like playing Nasus and every time I see him he fails real bad except with me ofcourse. I am going to create this guide for him.
I hope you enjoy and can help me to improve in the near future.

as the title says we depend on high amounts of damage, dealing it fast and then eating our victim for our own healing. for example once I was versus the other teams melee-dps + tank and there Nasus ( which wasnt much of a threat ) yes they killed me but this was the result only there tank walked alive I got 7K damage within 7 seconds while only have 3k when im at full hp.
so now I would like to explain how this could happen.

A one versus one fight:
I always try to come from the enemie side of the map since then if my target runs I have more chance to hit him more times. Then first cast my Spirit Fire so they are right on the edge of the circle so if they run back to base direction they have to go through the whole circel. Then I try to get the fight itself in the middle of the circel and cast Wither a fast as possible and first do a normal attack then right when it lands active Siphoning Strike because it will reset my auto-attack and thereby I attack a lot faster. now continue with attacking until my target is dead. In the case they appear to be stronger and have me at 25 to 20 percent of my max health I pop out my Fury of the Sand just to make sure I get the kill and also survive or in the case after my ultimate will not be enough it will give me a bit more opportunitie to escape.

I will try to keep this guide updated and if I feel it needs changes I will post them here.

10 feb 2011: Updated Items section with some more explanation.
11 feb 2011: Created another Nasus build To show a second way to play my guide but with the same goals/abilities.
17 feb 2011: Tested if life-steal would stack over 100 percent since I could not find it on internet written with proof.
19 feb 2011: explained a bit more in the summery section.

As I see most of the time in other guides there are the Pros and Cons so ill include them as well.

Pros:
High dps damage
your not going to die if they dont have a lot of stun-locks.
fairly easy ( in my opinion).
There are not so much player who play Nasus to be very strong.

Cons:
many time you have to tell your team in advance you will be the carrie.
you will have a pretty though time during the first 5 minutes of the game.

Greater Mark of Desolation
there is just no other rune that would improve my damage output more then this one.
plus I have a much easier time when im versus a tank.

Greater Quintessence of Desolation
same as with the Mark of Desolation more Armor penetration means bigger damage.

Greater Seal of Shielding
just to get that little extra defense to magic you gonna need.

Greater Glyph of Shielding
same as Seal of Warding only here the per level is already higher after lvl 10 so will be almost double when your max level.

I have to admit I am only level 22 myself but I picked the things I thought would be most usefull.
Offense 25 - Defense 4 - Utility 1
Since im my focus is with DPS im putting as much in offense as possible. when I will have 5 points left. which will go in magic resistance and upgrading my heal spell. and the last is just laying somewhere It wont be negative for me not really needing it.

If I follow this path for Nasus I am going to focus on dealing criticals and that way with lower percentage of life-steal still get high amounts of health stolen.
First in camp:
You may find it strange i go boots of speed first in camp but I like to have more movement then the other teams champions at start. Further I actually never buy any potions since I can regenerate my life with my passive ofcourse and I dont use lots of mana.

First trip back:
In a good situation I am able to buy my Ninjas tabi and the Sheen.
the Ninjas tabi is same as with the runes to get that little extra defense you just need.
Sheen has an Obivious reason, its kicks my Siphoning Strike to the next level of damage.

Second trip:
If I did not buy the Sheen in my first trip im buying it now.
the priority now is to get Trinity force as soon as possible because you once you have this item you be game changing since you have a massive power-up some may think I should buy this later but that is the whole point you want this first cause it makes you the most powerfull 1 on 1 champion in the game most of the time.

Later trip:
now every time I am in town with more then 750 gold I am going to buy Averice blades it might seem foolish but I really gonna need the critical chance and because it is the cheapest way to get it. ofcourse the extra gold is a nice bonus. but you could stick with 2 and get a cloak of agility, but then you are going to be missing 12% crit chance after you have you infinty edge.
after I get 3 Averice blades I am going to get my Infinity edge usually my game is in its final fases and we have already won or when I have a weak team and we are on the losing hand after this item it can change the whole outcome in a way so the enemie will be scared to face me even 2 vs 1.

Extra:
when the game takes longer then normal I usually replace Averice blades and go for some Bloodthirsters but these can be other items if you like. but if you went with 2 averice blades you could take a Phantom dancer.

Now for the other way I am focussing on getting a very high percentage of Life-steal so I will get my full attack back in Life if I follow this path I will deal a bit less damage to the enemie but I will be almost impossible to kill if they do not stun-lock me.

First in camp:
this is the same as for the critical Nasus way take boots of speed.

First trip back:
also same as for critical Nasus I need to get my Ninjas Tabi and then go get a Sheen as soon as possible.

Later trips:
I am going to buy executioners callings the reason I take two is that I want to build my life-steal but I do not yet have enough money to buy a Bloodthirster and this way I already have 36% life-steal and with my passive this will be 56% ok so now I get half my attack back in health.
Now I want to go save money for my first Bloodthirster which will boost my power a hell of a lot and I want to kill a lot of minions or whatever creeps that come in my way to build the passive of this wonderfull item.

Latest trips:
If I have some money left at this point of the game I will finish my sheen to a trinity force to generally improve all stat that increase my damage.

Extra:
When I have an exceptionally long game I get a second Bloodthirster and which will give me 106% life-steal which means every hit I do I will heal 6 percent more then I do damage so if I deal like 200 damage with a normal attack I heal for 212 health( 200 * 1.06 = 212).
Ofcourse here is some proof:
With the math: ( 449 * 1.05 = 472 ) or ( 100 / 449 * 472 = 105% )
http://oi52.tinypic.com/2e1t83c.jpg

1: Siphoning Strike just to get it start building up the damage for later.
2-3: Spirit Fire so I can farm a bit easier and I probebly wont need to chase anyone just yet.
4: Wither only because I cant pick Spirit Fire but wont really use it much maybe on a lucky kill.
5: lvl 3 Spirit Fire this way mage-creeps will be 1 hit kills so easy last hitting.
6: Fury of the Sands well this is a No-Brainer giving some options to start getting kills.
7-10: Wither Now the fun and the killing wil start.
11: Ultimate and dont complain about it!
12-13: Spirit Fire this way an occasional wave of creeps will be dead in a second.
14-15: Siphoning Strike just cause there is no other skill left to pick.
16: Ultimate and dont cry about it.
17-18: last skill....

I just added this to explain quickly about summoner spells. I usually take Heal and Teleport
Heal well just as it says is instant about 20-30% hp from early to mid game. many players find this spell weak cause it wont help much in late game but well I can live with it.
Then Teleport gives me lots of possibilities some are: Escape - Chase - Crowd Control - Helping my team

other possible spells to take are:
Exhaust a nice secondary slow really nice I like the heal spell more but its my preference.
Ghost real nice to escape or chase but again you should take what you like.
Flash is a possibility but rarely usefull since Ghost is the better option then.
Ignite well if you have much trouble with champions getting away with 5% or less hp then take this.

The one thing with Nasus has that most other Champions do not have. Is that you should always try to push hard and battle at the other teams tower and try to not get into fights. whenever the artillery wave come you should be at the other tower to get it down easier. And then in the occasion you should be alone with you own creeps at the enemie tower you should go ahead and finish the next wave of the other teams creeps so you can keep destroying the tower.

To conclude it a bit if I go with the critical hit life-steal Nasus I am dealing a much greater amount of damage which sometimes is nice and I choose this way whenever I have to deal with many champions who can stop me attacking with a stun or otherweise. Then the High-life-steal Nasus is very good to have versus carries and tanks since I am not going to lose any health when I am attacking. ( with this build I can also solo the Baron Nashor pretty easy and then after it have 90 percent health left. )
So I explained the way I play Nasus hoping everyone can give it a try and if you do not like it give me feedback on how you should change it. but however if you like it ofcourse which is what I hope please subscripe to it since I want to go expand it into a perfectly detailed guide.