Nasus is basicly a straight foreward DBS tank bastard, though there are two VERY important things you NEED to accomplish early game in order to dominate your mid/late game.
First off- DO NOT INITIATE UNLESS THE JUNGLER IS THERE, you need to last hit every posssible minion and farm your Q like there's no tomorrow, and you need the extra gold.
Second off- You go oom EXTREMELY EASILY, so DO NOT cast ANYTHING but your q, unless it is to cover a getaway.
Some other things to keep in mind is that it took me quite a while to get minion farming down to a science, so patience. Also it is a very good idea to take blue buff as often as possible, your mana hungry all game, and the CDR helps you spam your Q like there's no tomorrow in fights.

Though nasus IS NOT A TANK, you will become very tanky, and most likely be focused after the enemies have seen your massive dps. Anyways you ALWAYS communicate who you will focus in team fights, and when you initiate, use W on them, it'll slow them down, use your E and place it so they'll run through it or into your team, it lowers armor and has damage output, use your ult, to cause dps to everyone, and spam Q on them AS MUCH AS POSSIBLE. Sheen/Trinity Force are CRUCIAL to nasus. The Q will do an immense amount of damage, as the ult takes a percent of health away from whoever your killing.

Sometimes if there low enough, and by sometimes i mean AT LEAST once a game (ranked or not) someone escapes with near no health. no if you've been playing your lane correctly, you haven't used your E yet. Now the funniest thing ever is to E AND W under the turret so they either run out of it the long way, taking more damage and probably dying, or they have to run towards you, in which case you Q them. Now if that doesn't kill them, then ULT, and Q until they die, then flash out, if you have a flash.

WARNING-THIS IS NON-NEGOTIABLE
YOU MUST take Clarity/(Flash or Ghost)
You will start by NOT buying boots, so you will have no mobility without either flash or ghost early game. Nasus has a HUGE mana hunger mostly throughout the game, so clarity is vital as well. i promise you NEED the spells.

PROS- Nobody plays him.
Tanky as hell.
LOTS of unexpected DPS.
INSANE sustain.
Last hits REALLY WELL (with q, or e)


CONS-no DPS early game.
gets focused down late game.

I start with regen pendant because that And the innate lifesteal nasus has makes his Sustain in solo top unmatched. That does however leave his mobility lacking SO I NEVER INITIATE UNTIL I HAVE BOOTS. I still try to hold of on initiating until after i have warmog's, because i can live through anything after lvl 6 if i have warmog's and ninja tabi/merc treads. Trinity force causes your Q to end up doing around 1k dmg late game, with a cd of only 2.4s with blue buff, it gets REALLY funny REALLY fast. Phantom Dancer and Nashor's Tooth make your basic attacks do stupid damage inbetween. Rylai's is a REALLY fun item. His AoE, E, withh now slow people, as will his W with some bonus slow. NO one will EVER get away again. Oh, and his E will do more dmg, and you have more health. nashor's tooth is sorta deferable, and only for funsies