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.