Introduction
It's the first build i post, so please comment and vote this build. Any feedback is welcome.
By browsing Nasus builds, i noticed a factor that always holds him back severely - his shallow mana pool. It's not uncommon at all to run out of mana, particularly in early-mid phase. I decided to solve this problem by adding 
Manamune to an overall off-tankish build. This solves the problem largely, and by increasing his pool even further with other items i increase his damage potential while maintaining an overall tanky demeanor.
Masteries
I go a tanky 1/21/8 build with the utility for the 5% exp boost. Not much to change here, since offensive tree holds almost no value. The 1 point - for exhaust mastery - for the early first blood or gank escape.
Spirit Fire usage
A word of note. Usually, 
Spirit Fire's high mana cost causes it to be used sparingly, limiting its use as a harassment as well as farming tool. With Manamune, however, it is a viable tactic to use it for harassing andzoning enemies as well - most melee champs will avoid fighting inside the circle, which you can use to deny them exp and gold. In particular, SF can be used to scare the enemy champ away from the turret while your partner (or you, if the enemy lacks the cc to keep you near) damages it. The only thing to care about is aggro you get if the enemy is damaged this way - remember to back away if the enemy's about to step inside the circle, unless your partner is moving in for the kill.
Items
Grab your early Meki's Pendant and two hp pots. Your first back should be when you hit 955 gold - go then and fetch Tear of Goddess and 
Boots of Speed. From there on, you can stay on the lane almost indefinetly, 
Tear of the Goddess rejuvenating mana while you lifesteal up your hp. Your next stop should be 
Mercury's Treads - Nasus is quite vulnerable to cc of any sort, so they help a ton. The next item relies heavily on the composition of the enemy team - if they're caster heavy, start building 
Banshee's Veil, otherwise get a 
Glacial Shroud or even full 
Frozen Heart before it. The game's usually over by that time, but, in case it isn't, go for 
Trinity Force. Round off things with a 
Spirit Visage, abusing the CDR and the little extra MR to lead the final, decisive push.
Spells
I prefer going with 
Exhaust and 
Fortify. Both benefit you heavily in early game, prior for a once in a day first blood, latter for easier 
Siphoning Strike last hits. Unlike 
Ignite, however, they maintain some usefulness in late game. Think of Exhaust as a 2-sec extension of your 
Wither, which also has some (if limited) effect on enemy casters, while Fortify makes enemies think twice before trying to turretdive you.
Other options include:
-
Clairvoyance, useful for more map awareness, throw it where you expect to see enemy champs moving in for a gank.
-
Flash, which can save your butt if you overestimate your tanking capability
- Ignite, primarily to shut down those pesky healers like 
Dr. Mundo or 
Vladimir
- 
Cleanse. Although you most likely won't get too much cc due to Banshee's Veil, it will help you use your ult to the fullest.
Spells to avoid:
- 
Heal. Your lifesteal makes it obsolete and while you dont have a ton of hp, it doesnt make that much of a diffrence.
- 
Clarity? With this build, you will never burn through all your mana. Useless.
Tactics
Early Game
Focus on farming and last hitting. If possible, try to help your lanemate fetch an early first blood at lvl 2, but don't go all the way to get it - you can easily farm up the diffrence. And you can't if you're dead. Try to draw the enemy champs' attention away from your more squishy mate - although not quite a tank, you're most likely the tougher one. If the enemy harasses you too much, don't mind hugging the turret - with lvl 3 Spirit Fire you'll grab most of the gold anyway, and the enemy will most likely never get a single shot at your turret.
Mid Game
After you finish your Mercury's Treads and possibly Manamune, mid game begins. You're not as tough as a typical tank, so try playing a little more careful than usual. Your ult helps you survive a lot, but always remember, it doesn't make you immortal, and since you're not so tanky, you'll get focused a lot while on it. Try to keep last hitting mobs with Siphoning Strike whenever you can. As game progresses, you'll gain increasing bulk - use it to surprise enemies, expecting you to remain as squishy as you were, and allow your team to punish that mistake.
Late Game
By the time you finished Frozen Heart and Banshee's Veil the late game's most probably begun. You can take a whole lot of punishment, which makes you a great initiator, especially that your Spirit Fire allows your AD carry to take down the characters threatening you very fast. Try to use your ult sparingly though - bulky as you are, by ulting at the right moment you once again give the enemies a nasty surprise. Yet, although tanky, dont forget that you still pack a nice punch, especially when Trinity Force is a factor. Abuse it whenever possible.
Build Guide
Discussion (4)
Greater Quintessence of Health flat HP quint and Greater Seal of Vitality per lvl hp seals, increasing Nas's defences further with Greater Mark of Resilence armor marks and 
Greater Glyph of Magic Resist MR glyphs. An alternative, if you want a more offensive approach, is 
Greater Glyph of Scaling Cooldown Reduction CDR per lvl glyphs and 
Greater Mark of Magic Penetration m.pen or Greater Mark of Desolation armor pen marks, helping Nas deal dmg with ulted AD or Siphoning Strike, respectively.
