I have noticed that most of Nasus builds does not point out very important thing - managing q farm as early as possible. I'd like to present my build I have been recently succesful with. This build requires solo, preferably passive lane. It focuses on a lot of defense with addition of well farmed q damage.

In most games you'll be counterpicked if enemy team hasn't picked their top champion yet, you should go this build only if you're counterpick to their lane. Nasus gets most benefit of passive lane of all champions. This is awesome choice against tanky champions without continous burst damage. Pick this build to counter such champions - Cho'gath, Signed, Maokai, Nunu. It will work against other champions aswell but against those you get most benefits.

This build focuses on early cooldown reduction. Thanks to it you will get as much q farm as possible(remind how many times you had to lasthit minion without q becouse it was on cooldown). You get defensive/cdr runes and masteries. Your core items are:
Shurelias Crown - it gives you a lot of cdr, mana regen to maximize lane sustain and awesome active.
Sheen/Trinity Force - it's passive is a must on any Nasus that wants to be effective
Boots that gives enhanced speed 2 or more - Nasus needs that movement speed becouse one of his main weaknesses is potential to being kited/chased (aka no escape mechanism). I prefer defensive boots becouse you'll most of damage of your q - actually your personal chocie fits best here.

Those are core items (buying kindlegem abd regrowth pedant early is crucial), rest of build is situational - If the enemy team has farmed, good mage and their ad champions are rather weak you might want to buy FoN and some BF sword based items (bloodthirster synergies with your passive).

Early game

Basicly, you want to engage only if your jungler is ganking, or you see an opportunity to get clear advantage - keep in mind, every q cooldown you miss is 3 damage less in lategame. Max your q first and w second - use w to get safe out of their ganks. Don't hesitate to use summoner spell to get out of enemy gank.

Mid game

If you have picked teleport - use it to comeback to lane straight from dragon/blue teamfught or to get to it. If you need mana for q, push your lane with e, recall and come back normal. Try to farm as much as possible.

Late game

This is where Nasus shines - don't think it needs a lot of explanation. Just use Shurelia, press r and spam q to win. You should have over +500 damage on your q and thus with trinity you should be able to twoshot carries. Keep in mind ad carries are afraid of you, use it to gain advantage.