Nasus is a great champion, in my opinion, and his late game strength is heavily dependent on being able to farm effectively with your Q. Top lane, that means he's easily countered by any ranged character, and as a jungle, you're pretty much at the mercy of the enemy's counterjungle ability for the first five or six levels.
Champions that can outsustain him in jungle and still make it over to yours during your extremely slow early clear speed can make it a bit of a pain in the ass to keep playing, as being behind with Nasus ends up hurting twice as much, both via damage output with Q and by making you easy prey for ranged champions with lower defense than you may otherwise have.
That being said, once he gets rolling, all you need to do is build tanky items, and maybe something late game as a damage item to give your attacks that extra oomph. Boots of Swiftness are my preferred boots with the Alacrity enchantment, as he already moves fairly quickly, and this helps him catch up as well as move through the jungle that much faster, while somewhat mitigating his weakness to being kited.
I almost find Wither to be a defining skill moreso than his Q, as it slows the enemy to a crawl at later levels, and takes effect over a long time, giving plenty of opportunity to go up and smack them. His E is a farming skill for the build; nothing more, nothing less. It serves no purpose other than to enhance the damage of your Q further, and make early farming a bit less painful.
My preferred build is as follows:
Masteries: Defense most of the way. Notable slots are Swiftness, which stacks with Boots of Swiftness, Oppression, which is good for ganking, the flat armor bonus (a must for jungles), and Tenacious. I grab the CDR and Butcher from the Offense tree with the spare points.
Runes: I have a generic tanky jungle runepage which includes attack speed reds, CDR blues, armor yellows, and movespeed quintessences.
Early Game:
Machete and red potions.
Build Quill Coat as soon as possible.
Grab boots when you can.
Finish Spirit of the Ancient Golem
Grab Boots of Swiftness (with the Alacrity enchantment)
Build Sheen.
Grab Chainmail, and possibly Negatron Cloak if AP characters are starting to hurt.
Finish Iceborn Gauntlet.
Build Warmogs.
Build Atmas. If I'm farming well enough early on, I might pick up the Avarice Blade earlier.
For the Negatron Cloak, I leave it up to discretion on how the game's going (if it managed to make it this far). If you're doing well, and feel tanky enough, make it into a Mercurial Scimitar. If not, Banshee's Veil may be more beneficial.
Extreme late game: If you're not one-shotting their ADCs by this point:
Sell Boots of Swiftness, buy Merc Treads with Alacrity. Anything you can't catch with Wither by this point is something your teammates will have to deal with.
Sell Spirit of the Ancient Golem, build what you need to make up for on your defenses. As mentioned before, Banshee's Veils is a good choice, or maybe Spirit Visage to make up for the CDR loss, but always leave the build open for exploiting weaknesses of the enemy, or covering up your own.
Of course, it doesn't turn out like that every game, but it works well enough for me as a jungle. I haven't top laned as Nasus in a very long time, so I can't really say what works there. I think the semicarry potential of someone who, like Veigar, can one shot anything but a tank late game due to his scaling mechanics, has put me off of most other jungles, despite his early weaknesses and easy counterability.
In public games, you're unlikely to find someone who is able (or willing) to counter jungle, as most will tend to stick to their own areas until they get the level or skill that lets them gank, and they'll tend to leave you alone in the jungle. Security through obscurity; if they don't see you on the map, you might as well not exist until you actually become a threat.