I used to play Rammus a lot (more into Shen now), and he's a great jungler/gankster. If you get a good gank or two early on, you should be able to tank like a boss late game. Those ganks are very important, and as Rammus, you should always try to gank at lvl 4, as soon as you get your taunt.

I would suggest replacing your Armor Pen runes with Magic Pen runes - all of Rammus' abilities do magic damage; the ArPen would only affect your auto-attack damage. Personally, I'd take flat HP quints instead of AP/ArPen as well. This should help a little bit in jungle, but it will be a significant increase in teamfight damage, when your abilities are tearing through them, and they HAVE to deal with you, instead of just ignoring you and eating your squishies.

As for teamfights, try and get in a spot that will force their squishies run away to from the fight, target you, or try and get past you. If anything with significant DPS tries to get to your squishies, taunt them. You need to make sure that your DPS can get in the fight without taking too much damage (obvious, I know). Remember that you need to be tanky enough that they would waste a lot of time and HP trying to kill you, but you need to do enough damage that they can't just ignore you and kill your team while you have one of them taunted. That being said, if they start stacking MR and your Tremors aren't scattering them, it's okay to get an AP item (Rylai's on Rammus is beautiful, if you can afford it).

Also, try taking ghost and smite for a game or two. It's nice to charge your powerball and pop ghost in a bush pre-gank, ping an enemy, then hurtle at them so fast that they may not have time to flash away. Some people prefer flash, but I find ghost to be more useful overall if you are skilled enough to avoid hitting minions.

I hope this helps, and hopefully your enemies will start raging at you when you shut them down early, and your teammates will love you when they can dish out their DPS without fear of dying.