I figured I would post this to help anyone that might be having some issues holding threat on every mob/boss without problems. I feel the need to especially after running Toto Rak last night and tanking every pull and even the bosses as Conjurer.
When Pulling trash there are usually two or three mobs in a group. Two mob groups are pretty simple, as you can just Shield Lob one and Provoke the other (when you get it). After they gather on you, immediately use flash and then do your Fast Blade > Savage Blade combo on your primary target. After that feel free to use flash again and once the first mob is dead and you begin to focus on the second, you can do the FB > SB combo once and then use FB > Riot Blade to regain MP until it is dead.
Three mob groups can be tricky at times, especially when one of them is ranged. Lob/Provoke one monster and then pull the others to the ranged enemy and use flash. FB>SB combo your primary target and flash again. After that you can FB your primary target and then SB another target to get a big threat increase. Do this again for the other mobs in the group and use flash once more and nothing will ever stop attacking you. When your group starts focusing the second mob, toss a quick FB>SB combo on it and then feel free to spam FB>RB until the group goes down or your MP is full. With this sort of rotation you can hold threat on a full group, even if your party members are attacking separate targets. Sometimes I will Lob one monster and provoke another in a group of 3 for the added threat, however you can also save provoke for any stray/roaming mobs. Once you know the dungeon you will learn where these situations can arise and plan your cooldowns accordingly.
I have only partied with two THMs while playing, but if you have one in your group that has Fire II then you will need to spam flash much more. With their dps being spread out over the whole group, your primary target will be a bit less likely to stop attacking you, so flash spam will be more than enough to keep it's attention. Note that you will want to flash at the start of the fight, then again as soon as possible, and then every time they cast Fire II. You can switch to other targets for a Savage Blade, but it isn't really needed with more flash spam. You can focus more on using Riot Blade to regain your MP.
Bosses are pretty simple to keep threat on. Just pull with Provoke and/or Shield Lob, get into position, pop a flash, and then start your FB>SB combo. If the fight requires movement, feel free to toss an extra Lob or Provoke when the monster is out of range for more threat. For those fights that have adds, have your ranged party members stack near you but not in front of the boss so you can easily flash spam to pick up the adds. Try to remember the timers for the bosses abilities, as they usually use them in set intervals. If you think the boss is about to do a frontal AoE move, wait until it does it before trying to reposition it. And as I'm sure most of you know already, pretty much any move with a charge bar can be sidestepped/ran out of so be sure to be on your toes!
One other topic I want to hit on is Cross-Class abilities and Defensive Cooldowns. You will want to try to stagger out your abilities as much as possible to maximize deamage reduction. I suggest taking Featherfoot and Second Wind from PUG, Keen Flurry from LNC, and Foresight from MAR. Keen Flurry and Foresight work well together. Featherfoot is great for large groups of adds or fast hitting bosses. Rampart is easily spammable and is the only ability we have to reduce magic damage, so it works well on casters. Convalescence is best used when your hp is around half and your healer might be having trouble healing up you or the rest of the group. It works great with Second wind and possibly potions as well. Finally, Fight or Flight has a long cooldown and minimal effect for us as a tank, so I would save it for the start of boss fights or in situations where you need higher single target threat. Note that I didn't take Cure or protect as cross class abilities. This is because cure is nerfed so severely as a CC ability that it isn't worth using. Healing for 50 hp when you have 800 is going to be detrimental to your tanking. You won't need protect or raise in a group instance with a conjurer around either.
Some UI changes might be beneficial for you as a tank and even a healer. For me, the Target bar was too far away from my character/hp and seeing the charge bar for abilities was far too hard, so I moved the target bar down above my HP/ hotbars. Moving the party window may also prove beneficial to you, as it is a bit tedious to have to look to the top left of the screen to see the tiny threat meters on each person's class icon. I have mine to the right of my cross-hotbar, al la FFXI style.
I guess to sum this whole thing up, Use Flash liberally, yet smartly and smack non primary targets with Savage Blades. Regenerate your MP on the last mob or two of each group. Be very aware of your surroundings and try to constantly zoom your camera out/around when you are in a new dungeon. Also, don't be afraid to tell your party members to attack your primary target more or to stack on you when adds pop in a dungeon. Typically adds will beeline it straight for the healer, so be prepared and ready to peel them off.
If anyone has any questions or comments, feel free to ask/say them. I don't know how Marauder tanking works yet, but I am sure you could use similar tactics to the same end. Hopefully this will help some of my Fellow GLAs become pimptastic tanks as well! :D