Early game:
Nasus is not a bad early game hero, he can keep opponents away from gold and xp with SF, giving him a lot of lane control. This is a lot harder when you are up against ranged heroes. But when placed good it will give them a hard time farming too.
It is VERY important to last hit with SS, otherwise you will be nothing late game. This is a critical stage in the game for Nasus because of the last hitting. If you can do it well, you'll be able to deal good damage with the skill without even leveling it that much. This is what you want because SF is a skill you have to level.

Mid game:
This is usually the stage where the ganks come in. And when I play Nasus I like to push a lot to give your team a great advantage later on. Nasus is a great pusher, he can wipe out a creep wave with one spell and can damage a tower quickly. SS works on towers, so use it when you're hitting a tower to destroy it quickly. However, all this pushing will atract a lot of attention. Thats why I take cleanse. Its one of the best summoners skills around. You get stunned? Cleanse out and walk away. It will often save your life.

Late game:
This is where your ultimate begins to really shine. Its nice at the beginning but deadly when up against 5 champs. They will take a lot of damage and the chaos will get them to flee, after which you can easily hunt them down. Any champ with good AOE spells is a good team champ, and Nasus excells at this.


Items:
I start of with a rejuvenation bead and a fairy charm for good health and mana regen. Use these items to make a Tiamat, which gives you all the minion kills, getting you a lot of money. And money is what you want, because expensive items will follow. Getting a Sheen is a must for Nasus. When you activate SS, Sheen will also be activated, giving your SS an 80% boost. Changing Sheen into Trinity Force makes this boost even better. Trinity is a great item not only for the 130% boost on your SS but also the chance of slowing with your basic attack and Movement speed and Crit Chance will help you out a lot too. When you have Tiamat, Greaves and Trinity its time for some raw damage: Black Cleaver. Hitting enemies with this will reduce their armor even more after taking armor reduction from your SF. A lot of the time they will end up with negative armor, taking increased damage from physical attacks, in stead of reduced damage. After all this armor shredding its time for criticals, and we all know what that means: Infinity Edge. You will end the game with about 40% crit chance. And when you get a critical SS you will be amazed by the damage you deal. With the crit runes you'll deal even more. So you have an attack that deals about 200-250 bonus damage, (depending on how many minions you have been able to farm with SS) 130% bonus damage from the trinity, while critting for 280% your AD. Its like a good nutshot! After the Infinity I usually take Bloodthirster for more lifesteal and AD. I rarely get it because the game doesn't last that long, but its a really nice one for Nasus, giving him 45% lifesteal, making him pretty much invincible 1 on 1. And its really easy to regenerate, just go into the jungle for 10 seconds and come out full health. It's also possible to take banshee's veil if you die too much. It will help you survive and you have a lot of damage output at this point anyway.