I'm a newer Nasus player who is just starting to get a handle on him. I've been playing him primarily as a tank. Get last hits constantly with Siphoning Strike and otherwise conserve your mana and stay in your lane for the first part of the game. Early rank 2 or 3 Wither can be used to save someone or help a gank. You shouldn't need to use Spirit Fire to farm early, so it is only used to help damage/debuff while they're withered in a gank.
Later on you'll make a much more consistent use of both Spirit Fire and of Wither. Mid-game Spirit Fire can be a great farming tool (Make sure you're still finishing things with Siphoning Strike though) as well as a powerful ganking tool. Don't forget that the armor it removes can be more damaging to an enemy champion than the actual damage it does.
Late-game you're a tank. You get in first, stay in as long as you can with Fury of the Sands up and dealing damage with your cloaks, Siphoning Strikes that hit like a truck, and Spirit Fire. Don't be afraid to duck out when you need to.
Summoner Abilities: Heal + Cleanse
Runes: Health or MR/Armor for Yellow, Health for Black. Red can be Armor Pen, Critical Strike Chance, or Damage. Blue there doesn't seem to be a really ideal choice. You could go with Mana Regen or Cooldowns for primary... or you could just go for more Health/MR/Armor.
1. Siphoning Strike
3. Spirit Fire
6. Fury of the Sands
7. Spirit Fire
8. Spirit Fire
9. Spirit Fire
10. Spirit Fire
11. Fury of the Sands
13. Siphoning Strike
14. Siphoning Strike
15. Siphoning Strike
16. Fury of the Sands
17. Siphoning Strike
1.) Regrowth Pendant
I find in medium/high ELO games that this adds a lot of consistency. Getting a Meki and 2 health pots is nice, but it leaves you open to harassment early. Being unable to fuel up your Siphoning Strike leaves you in a weak place for the rest of the game.
2.) Upgrade Regrowth to Philosopher's Stone and get Mercury's Treads
The Philosopher's Stone early gives you the health regen and mana regen you need, and it also provides a bit of income. I could go with a Chalice here instead for mana... but it seems like it is overkill for the mana Nasus needs. The Mercury's Treads can be replaced with Ninja Tabi if the enemy team is much more physical attack oriented.
You'll be upgrading this later, but for now it will help you wreck people with Siphoning Strike as well as have a bit of an easier time farming with Siphoning.
4.) Banshee's Veil + Sunfire Cape
In whichever order you feel you need. These significantly contribute to your health and your armor/MR. By this point you should be getting to the higher ranks of Spirit Fire, so you can clear entire waves very quickly with your Sunfire + Spirit Fire. Banshee's + Mercury's + Cleanse makes it really ridiculously difficult to stop you.
5.) Upgrade Sheen to Trinity Force
You can do this by snagging Phage first if you're already needing to make a trip back. This item was practically made for Nasus. The 50% stronger Phage effect makes your Siphoning Strikes hit really really hard, the movement slow combined with your counters to CC, high movement speed, and wither allow you to keep almost anyone from escaping. The extra crit plays in nicely with the already devastating Siphoning Strikes, and the extra health brings you up to 1300 from items so far.
6.) Aegis of the Legion
More Armor, more MR, more health. More importantly, however, is that it also gives some survivability to your teammates.
7.) Warmog's Armor or Guardian Angel.
If the game lasts this long then you'll have to decide on which one you want. They're both good choices.
Anyhow, been having pretty fair success with this build. It farms better than a lot of the tanks, is very durable and hard to stop, and with sheen/trinity combined with Siphoning Strike it packs enough of a punch to wreck DPS-heavy champions in the middle of a team fight. It also does very well as a chaser, and the damage from Fury of the Sands + Sunfire Cloak + Spirit Fire is considerable.