From my experience, Nomad does best with a level advantage. If he can solo, that's preferable. Mid works well.
What good Nomads end up doing is using their early game power to decimate the lanes, be they his or the side lanes. His speed and mobility lend themselves well to being able to move about the lanes (and maintain rune control!).
In spirit of this, boost his burst damage and mobility. Get Phase Boots early to help with both.
Later on, your items choices get varied. Some people really like going for a Helm of the Black Legion to keep up their survivability, then going for pure damage. Others get themselves a Frostburn and go from there. Nullfire is an excellent choice on him.
Just remember that Nomad is hard to play, and requires a great deal of team support in order to shine. You're going to have to get used to dishing out orders if you're going to play nomad.
TL;DR: conquer early game through a level advantage and keep owning.
Nomad works well both solo mid and in a side lane. Rune control is very important for him to pick up kills when ganking side lanes. Double damage will allow you to kill int heroes a couple levels below you in a couple seconds; haste is also very useful on him; illusion can allow you to safely tower dive.
Nomad is a great solo against other melee heroes, as his passive allows him to come out on top if auto attacks are exchanged. Make sure to charge it by moving around a lot between the times your opponent moves in to last hit.
Against ranged heroes, make sure to pick up a buckler and a lot of regen; manna pots work well if you want to keep spamming mirage strike to harass. You should always keep enough manna, however, to use sandstorm or mirage strike for an escape mechanism. If your opponents have a lot of aoe stuns, make sure to use mirage strike to escape.
In terms of items, it all depends on whether or not you are being targeted in skirmishes. I really like frostburn because it increases mobility, dps, and hp. These are all things Nomad can put to use well. People like to use nullstone on him, but of course that is situational depending on your opponent's team so I wouldn't call it core. The same goes for shrunken, it can be very powerful depending on the other team's lineup. I don't really like using basher on Nomad unless you are trying to carry and end up fighting the other team's carry a lot. Then again, I really hate RNG items in general.
Perhaps the most important thing to remember when picking Nomad is that he isn't a hero that works well in most lineups. He isn't a great carry, he can't initiate, and he has no disables. When I first started this game, I really enjoyed Nomad and made the mistake of picking him at the beginning of nearly every game (hence my large %). More often than not, especially in lower brackets where nobody else cares about team composition, we'd end up with a team that lacked synergy and got raped mid/late game.