Here are 3 tips from Proxy for players who are in Nightmare making their way to Hell. (And earlier players as well.)
1) If you are leveling Black Smithing(BS) I HIGHLY suggest that you stop. You will of course want to level this later, but as you progress through the game your gear is going what is going to either hold you back or make you shine. Leveling BS is a huge gold sink and the chance that items you can craft will be useful is a crapshoot. I would save your money and buy things off the AH.
As volatile as the market is right now, you can still find useful items for really cheap. Once you get to Hell, you are going to need a nice little chunk of savings. I bought around 4 pieces of gear and they cost me 90k. It was necessary though, it increased my Vitality and DPS to a level which makes Hell manageable. (I will say this, DH's who have decent DPS, you will rock through Act 1 Hell)
2) So, now that gear is covered I want to cover your build. I am not saying that whatever you're using right this second isnt a good build, but I am saying this, you will need some type of knockback. I am currently playing a Monk which I built as mainly an AoE tank. Once I had beaten NM we were helping a friend level to 50 so that he could also come to the Hell difficulty with us.
While doing this I experimented with some things to make myself more efficient at kiting and taking as little damage as possible. Deadly Reach, and Wave of Light became my best friends. In Hell I can absorb damage from about 3-5 enemies without much hassle, keeping myself healed the whole time. Any more than that and you reach a problem. The mobs hit hard, and you dont have enough life to save you.
Barbarians seem to do fine with the incomming dmg, so they are the only class I dont think needs ranged abilities.
3) Finally, team up. If you can find a group, join one. This game can of course be done solo but each character has a weakness and as you go on through the game, enemies will exploit it. Find a group, use your synergies and progress.