I'm not a seasoned raider but I have one solid piece of advice when it comes to raiding I want to share and that is: if you wipe on a boss 4 times and there is no progress in sight (i.e. you are getting closer to the kill every time) move elsewhere or try again another day.
Note that I don't say that you should just try 4 times and then give up as a general rule. Sometimes it takes more than 4 tries to get everything right or you realise tactics was all wrong and start over again. But the keyword here is progress. When you wipe repeatedly, going through the same motions and the health bar of the boss seems stuck at the same percentage every time you all die, chances are you will continue wiping until one of two things happen: either you have to call it off three hours later after continued wiping, leaving everyone feeling exausted as well as disappointed (more likely on guild runs) or the group will start desintegrating with people blaming each other and leaving in a huff (more likely in pugs).
Sometimes all it takes is a short break, killing something else. Naxx is a good place since, if you haven't cleared the rest of the place already, you can always move to another wing for a change of scenery for a while and then go back to annoying Heigan, irritating Gluth or the four tiresome horsemen and you will probably kill them.
Yesterday I didn't follow my own advice in Ulduar. We tried hard mode on Flame Leviathan (and no I don't agree with Tamarind that it looks like a golf caddie, I always think of it as the mad snow plough. There is after all a lot of snow in Storm Peaks. Perhaps Flame Leviathan was set to work though it all and went insane by the sheer impossibility of it all) but that was too much for us to take on with fire, ice and all the crazy flowers running about so after two tries we destroyed two of the towers. Two towers up we usually manage perfectly fine but yesterday was one of those nights when things just didn't work. We died again and again and stubbornly refused to destroy the two remaining towers because we should be able to to it really. We wasted a lot of time on the evil snow plough and in the end we still destroyed the other two towers just to be done with it. By then it was quite late and I should have gone to bed but it felt so depressing to end the night like that so I said ok, let's have a go on XT before bed. We went to the scrap yard, killed the boss in no time and that was it. It went so fast I felt what the hell, let's do Kologarn too so we went over to him and yep, he went down, no problems at all.
If we hadn't been so stubborn on Flame Leviathan we would have had time to move further in Ulduar. As it was now at least we felt the night ended better than it begun but still I regret we spent so much time on Flame Leviathan with towers up just because we should be able to to it really.