In an earlier post, I listed some of the fundamentals of tanking. The first thing I listed was perception, your awareness of what's going on. This is especially vital in Ulduar. In this regard Deadly Boss Mods is a godsend. So much so that I have made it required for all raiders in my guild.
Ulduar is a true test of a tank's skill, in every position. There are fights that require a lot of movement, like Ignis. There are fights that test your ability to recover from disruption, such as Auriaya. There are fights that test your coordination with your healers, such as Mimiron and Vezax. There are fights that test your ability to quickly pick up and establish threat on multiple adds, such as Thorim and Razorscale. Nearly all of the fights have something that you shouldn't be standing in.
Your UI setup is one of the keys to aiding your perception in these encounters. Having a clean UI that allows full view of your surroundings is of paramount importance. Keep the chat log open, so that you can catch any DBM prompts that you might have missed. A conveniently placed threat meter, such as Omen, is also a necessity. Keep things that you deem important near your character, so you don't have to look away as much to get information you want. I personally use IceHUD to bring up my health, mana, and debuffs, and those of my target. I would recommend it to any aspiring tanks. The necessity of raid frames depends on the organizational skills of your raid leader. I always make sure that the healers are all in group one, and that tanks and any other needed personnel are in group five. I'm in group five, and pull group one out using the default raid frames. In every fight, you will want to know the disposition of the healers and the tanks at a minimum. There will be other people who you'll want to track in specific encounters, such as the harpoon chucker on razorscale, but the healers and tanks are non negotiable.
This brings me to what you actually do with this setup. In any fight there are two states. There is a dynamic state where you do not have control of the situation. This is usually during the pull, when adds spawn, and when a boss must be moved. Then there is a stable state, which is when you have established aggro, gotten in position, and all is good.
During a dynamic stage of a fight, you will need to focus away from your character. Locate the mob that you need to pickup or find the next spot you're moving to, those are your priorities. If a void zone, rune of death, AOE of general badness spawns under your character, it's no big deal because you're going to have moved out of it before it ticks. However, if one of those spawns where you're heading, if you don't see that, you'll run into it right as it ticks. Your goal in dynamic stages are to gain control of the situation as quickly as possible and return it to a stable state.
During a stable state you should be focusing on your character's feet. If you're starting at the amazingly detailed boss model that's about to stave your skull in, you are wrong. Keep your eyes on the floor, always on the floor. Besides, odds are you can only see their crotch anyways. Don't be the tank that eats a void zone because you were too busy trying to look up Kel'Thuzad's skirt. Ensure that your character is not standing in something you should not be. Keep an eye on the DBM prompts so that you can anticipate and plan for changes in the fight as they come up. Your goal in the stable state is to be ready to survive the transition into the next dynamic phase of the fight.
Tanking in Ulduar can be a lot easier than it seems, if only you know the right places to look.