Foreword: I wrote this tutorial in Italian and then I translated with Google, do not break my **** for various errors.

I wanted to write this guide because nasus is my main and I play really bad, this thing pisses me off because it is a really simple, so much so that is classified as "recommended".
There are few things to keep in mind to play it well:
is an off-tank;
-need time to strengthen the siphoning Strike, possibly staying alone in his line;
must-have style of defensive play and not be reckless (calm and patience are two virtues necessary).

The runes are in line with the concept of off-tank.
The marks are to help the farm's initial Q or armor penetration to make more damage in the late game.

A configuration offensive is ideal to make some kills early in game with the help of jungler or alone if the case permits.
Have a wild configuration wholly offensive would be pointless: recall that nasus is not a tank, but an off tank.

I want to say a few words on the first items that, in my opinion, are crucial in a game.
The Shield: key to advance through the minions and to enhance the Q without fearing that the enemy we face is too bad.
+1 Shoes: useful to enhance the Q without being too much in contact with the enemy, but also vital to avoid the first gank.
Sapphire Crystal: useful for increasing the amount of mana, which is essential for a peaceful farm.
Sheen: fundamental piece to enhance the best Q doing a large amount of damage.
Ionian: given that the major source of damage to the Strike nasus, these shoes allow us to use it more often.
Phage: useful to be able to wander here and there among the minions with a good amount of life and a decent attack.
Trinity: FUNDAMENTAL object!

Now part of the chain armor. The order may change depending on the exacting, but the order of the objects to create them is not accidental: I followed a specific order in this guide and I recommend you follow.

As you have noticed but I prefer to enhance the Fire Spirit Strike because it allows me to eliminate large waves of minions that often arise near his tower, creating a double disadvantage: nasus great damage to the tower, and the ability to attack the opponent without being taken into account by too nasus, from the tower and our minions, all busy killing those enemies. Thanks to Spirit Fire you get rid of this problem quickly and taking care to target an area that also includes our opponent (this always happens), he also carries a decent amount of damage.
This problem can be solved by simply killing the enemy minions constantly because often the enemy and why is it that prevents jungler prevent our being to make good gank our line too far.

There is no other choice outside of Ghost and Exhaust. When nasus decides to make a kill, it should.
As teleport spells are useless: 300 gold ahead of them are critical and need to go for broke in battle. Do not ask yourself the question that if you use Recall your opponent destroys your tower because:
First-before you leave you leave your minions in a favorable using Spirit Fire of those enemies, and possibly, even when you are absolutely never in your tower, but try to advance their own line at the middle, between two bushes . So you'll have time to go without the tower suffers too much damage if the opponent is very aggressive at this time of your absence;
2-if your opponent is very aggressive and expect great damage to your tower, use Ghost to go faster (that's why I prefer it to a dual role: in combat and out of combat);
3-call jungler to cover your absence if it is available.

Use both spells during a fight to make sure you do not miss the opponent. Similarly, you can use to ensure an easy escape from a fight or a gank.

Important: used first and then the Wither Exhaust, so you can use the Wither twice taking the opponent always slowed down for many seconds. (This strategy works very well against its magic, and Gangplank W)

Other spells like Heal or Flash are really useless: the first because an off-tank does not need instant short trips, because it has no stun spells or "snapshots" with which to surprise the opponent, but is meant to be in melee long in combat. Heal is useless because of the small quantity of life than what you'll recover with an off tank.

As I said before, it is recommended to kill the minions with the last-hit back to hold the line to promote our jungler ganks. If we were to find crushed under its tower a good use of Spirit Fire will solve the problem. Do not be afraid to use it to kill minions in other situations (always aiming for an area that could damage your opponent), but be careful not to run out of mana you will always have enough to slow down, to use the ultimate and in case also to use a Spirit Fire.