Nasus was my second love in this game. I loved the power a q could bring after diligently farming and trading through the lane phase.
My standard runes are AD Quints, 4 AD and 5 Hyb Pen marks, Armor Seals, and MR Glyphs. Many people like to run CDR Quints or Glyphs, but i haven't had the IP to drop on them yet. For Masteries, i think i run something like 14/16/0.
My standard build is the tanky bruiser. The item path depends heavily on the match up, due to your lane phase being mainly farm focused. My most used build is the Triforce rush. I used to build Iceborn, but fell in love with the stats that Trinity gives you. Once your Trinity is finished, your farming and trading get flipped into over drive. After Trinity, it all depends on the enemy team comp. Spirit Visage is my go to MR item because of the CDR. Same with Frozen Heart for Armor. For health, i prefer to go Warmog's, because the passive is ridiculous, but you could also go Frozen Mallet to stick onto people and make it easier for your carries to catch them in team fights.
If i get fed in lane, i will build Randuin's or Spirit Visage, then build some more damage if i'm feeling saucy. Black Cleaver, Rav Hydra, and Bloodthirster are all amazing on Nasus if you get ahead. I've even built full Life Steal/Crit on Nasus before.
Match ups to look out for, anyone who is powerful in the early game. Pantheon, Darius (not as bad as Mantheon, but comes down to skill. Beware him post-6), Aatrox (if used properly), Jayce (though i haven't played him since the latest nerfs), Teemo (requires a different build/skill order), and i would imagine Morgana top against you would be hell for your farm. Basically, beware lane bullies and poke.
During lane phase, your job is to farm. Even if you ignore the enemy entirely, farm your ass off. Prioritize the cannon minions, as they give you double stacks. If you fall behind a bit, farm some jungle monsters. The big monsters at all the camps give you 6 stacks instead of three.
After lane phase, you can split push or join the team for team fights. If you choose to split push, don't ignore where the team is and try to go for the nexus on your own. Even at low ELO, you will only have so long on your own before they bring three or even all five to end your push. If you run TP, which you should, always keep an eye on what's going on around the map. The scariest thing when facing a Nasus is to see the teleport come in and get withered before the cane strikes half your health down.
In team fights, you are the front line. If you build tanky, you can survive decently well, but don't go running into their team. You don't have hard engage. The best you can hope for is to wither someone who's too far up. Once a fight happens, pop your Ult. Don't wait until you're low. It gives off AOE damage for the duration, on top of the bonus stats. Use your Spirit Fire in the middle of their team and try to get wither down on their carries. Use your cane every time it's up (which can be every couple seconds if you build CDR). The only down side to being the front line as Nasus, you will not be treated like a Sion or a Maokai. You bring the damage as well as being chunky, so it is actually probable that you will be focused. You may die in team fights, but you will distract the enemy from your carries, allowing them to clean up the fight.
Also, Infernal Nasus is the best skin in the game.