1.) Armor runes, defense masteries.
2.) Always ALWAYS get homeguards on him, no matter which boots you pick up (I like mobi for low price/high coverage) but homeguards+Q mean there is no penalty if you have to back, you'll be right back in the jungle in no time (speed boost wears off at 2nd turret) This means safe jungling, and less down time, it's pretty much gp5 in the pocket, late game it means you can defend split pushes like a beast.
3.) Do not underestimate the MR on your curl, you aren't JUST anti AD.
4.) Always take flash, not ghost (and not tp, cause homeguards). When you are ganking you have to be able to jump the minion wave or a body blocking ally, if you can't you lose a lot of your threat.
5.) Tank towers for your team, rammus' armor outscales turret damage at all points of the game after you have 3 points in curl.
-Early/mid game this means easy shallow turret dives if necessary
-Late game this means communicate with your team instead of pushing down mid against their entire team when you're sieging call your team into the jungle if you have vision control and go take a side lane tower, you can easily tank the damage and enemies often leave towers WITHOUT creep waves near them unguarded and usually arrive to late to stop a full team siege against an unprotected tower.
6.) Don't be scared. You're Rammus - If your team IS in position to teamfight, just go in and curl, if they blow all their cd's on you than your team is gonna win the fight.
7.) Sunfire is an excellent first item on him (After golem), speeding his jungle clear, amplifying his defenses, and giving him more presence in a team fight (since you want to be right in the middle, in conjunction with your ult, you will be pumping MASSIVE AoE damage. You can't let them ignore you.
8.) Banshees>Visage if need MR on rammus. It makes it that much harder for them to keep you off of their precious fragile things they keep in the back.
9.) Abyssal/Sunfire/Thornmail if you just want to have fun and are way ahead, All of those items amplify eachother. Sunfire Aura/Tremors/thorn damage all effected by -mr aura, and you still get to stay tanky... just not as tanky as you are when you are being superseriousrammus.
10.) Consider buying your mobi+homeguards with quill stone if you can't afford golem on first back. The gold generation of simply being in the game more often, and the extra presence is usually more valuable than getting an early kindlegem.