Personally, I avoid a second GP5 item on him, as last hitting is so easy when picked in the right match ups.
Mana management is not hard if you refrain from using "E" until you really mean business. It does almost nothing before level 3, and you don't want to push your lane.
So I like to build glacial shroud early, with boots 2 for mana and MR, then get my sheen going on. Sheen is something like the equivalent of 25 "Q" farm to your damage, well worth it early. After that I like to build SV and finish my Frozen Heart. With those 2 items my CD is maxed (via talents) and Wither only has a 2-3 second CD before I can drop it again. Very potent CC.
I also like to run lifesteal quints, starts me at 20%...nice sustain.

You start using about twice the mana that you recover between Q's after a while. I haven't played Nasus lately to remember if it's level 7 or 9 that your mana starts dropping rapidly, but nonetheless without some form of regen other than Philo you will be dropping mana even if you only last hit with Q's. After Glacial's CDR, it gets even worse; I think it's around 4-5 waves to drain your whole pool with just Q's. If what you're saying is true, then they must have massively buffed his innate mana regen since I played him.

pick him when you see a yorick on the other team, then smack his ghouls in the face with your Q. you get a stack, and you dont push the lane.

i heard that tiamat was viable on him, how so?

My 2 cents on this matter...

I think triForce is overrated on Nasus. 4xxx gold for bare minimum stats, imo no thank you. If the object of the game is "farm Q" and the rules are rush triForce (first fully built item) then no wonder Nasus are underplayed and have to lane against specific champions. If Nasus depends on their Q damage, then get tanky first!

Skip the triForce all together (or get it as your final item). Run a rune page with over +20 to your def/mr to counter the champ your going against. Counter your lane with items that build up your hp, res and cooldown so you can farm under harass. Carry pots at all times! You don't know many times pots saved my life during a dive (both diving the enemy and getting dived by the enemy and their jungler) or sustaining harass.

Try a different mastery page. I was always thought to believe runes and masteries are suppose to help a champ early game out, optimize them to give you that advantage.

Nasus is a pharaoh and the minions are his people, if your getting harassed or planning on fighting make sure its happening near your minions because they will target the opposing champion. Remember minions OP!

Forget the 40min farm rule, its stupid. If your team can win and survive without your help for 40mins you might as well be afk. When the team fights happen get your butt over there and help the team, the cc you provide turns fights around. Also if you choose to ignore my feelings about the 40min farm rule then buy wards and ward all over, I know for a fact no team would let nasus farm top that long without paying a visit.

Pick Nasus against champs that need to kill to snowball. If they cant kill you, and they shouldn't then you have just gain a huge advantage. Don't be afraid to go for kills or harass yourself. Nasus is an alpha dog not a....cat. Assert your dominance, claim your lane by pissing on it or spilling first blood on it.

All I have to say for now, sorry if I didn't make sense or some jokes wasn't funny. Just trying to say there is no "set in stone" way to play nasus, and that mainstream nasus isn't doing so well right now.

I mean, your team needs you and all, but you should always be farming, barring your team needing you for something in another lane. if yours and enemy team are squaring off in mid, waiting to start a fight, farm the minions so your ap carry doesn't have to spend mana on them

the point of nasus is to build full tank, and still have craptons of dps. you need no damage items because you have other ways of getting damage.

The way i do nasus in this order is:
Boots+4pots
Philo or sheen (depends if you're farming without problems go for Philo, getting harrased go for sheen so you can outdamage)
Merc or Ninja (don't go the usual top countering way, go against a full team thought process)
Sheen or Philo whichever you didn't got
Aegis/Frozen hearth/Sunfire/Thornmail (i don't usually like thornmail, but against something like a Jayce top and AD jungler combo it's ok)
Trinity
Shurelyas
The other two slots are situational, if you need more offense go for LW, you need MR go for FoN/GA/or the usual AP item that builds with negatron cloack (Don't remember it's name), if you're getting cc in teamfight QSS, it really depends on how things are going.
Just remember that till lvl 10 you should always have at least 1HP pot and never stop warding.
Once you get it right run teleport/ghost since it will let you teleport bot and countergank, or go for dragon and return to lane to farm.
Another thing is to remeber that your Q+sheen proc does damage to turrets, so at some point you can just take them down in a couple of seconds.

You didn't ask for it, but i rum flat AD red, Armor yellows, flat MR blues and Lifesteal quints.

Don't bother playing Nasus right now. Sorry, but he's not viable with really any team due to the fact that if a top laner gets farmed and fed, they can carry a decently bad team, Nasus can't unless he farms forever and it's honestly super easy to shut that down if the enemy team even wants to.

I love Nasus and if you love him too, wait for the new items. Word is, Tiamat lets you proc and stack your Q on every minion, aka if you can 1 shot a whole minion wave with your Q, Tiamat gives you 6-7 stacks all at once. This allows Nasus to do mean stuff like counter-jungle or farm behind the enemy tower now which he previously couldn't do because he couldn't even wave clear due to wasting opportunities to build up your Q.