When should I use my CD's?

While tanking 5-mans in Blood, I'm having some troubles deciding when to pop my cooldowns (especially really short ones like Vampiric Blood and Bone Shield). Should Bone Shield be refreshed every time it drops? Vampiric Blood and Icebound Fortitude during high damage? When at low health?

Thanks for any input, I am very new to tanking.
Bone Shield should be used basically on cooldown. Don't forget to use it before you pull if possible. The charges can only be removed once per 2 seconds, so it won't immediately fall off when tanking multiple mobs.

Vampiric Blood should be saved for when you know you'll take a decent amount of damage. Use it right before you actually take that damage so your healer has an easier time keeping you up through that damage (or, if unglyphed, so that you have more HP to take that burst). Generally, if you tend to pull without CC, this will be a few seconds into the pull (casters have cast times, mobs need to run to you first, etc.).

For a DK, you generally want to use your cooldowns proactively as opposed to reactively. That is, try to anticipate the incoming damage (which just takes experience), and use the cooldown accordingly. There's a pretty marked difference between "wow, my health just hit 35%--I should probably use a cooldown" and "I know my health has a high chance to hit 35% in about 4s, so I should figure out what I'm going to do about that."

Whether that means you hit the cooldown before or after you take the damage (sometimes it's better, sometimes it's not; depends on the cooldown), the point is that you aren't taken off-guard by that incoming burst, which lets you think more clearly and not do something stupid.

Bone Shield should be used basically on cooldown. Don't forget to use it before you pull if possible. The charges can only be removed once per 2 seconds, so it won't immediately fall off when tanking multiple mobs.

Vampiric Blood should be saved for when you know you'll take a decent amount of damage. Use it right before you actually take that damage so your healer has an easier time keeping you up through that damage (or, if unglyphed, so that you have more HP to take that burst). Generally, if you tend to pull without CC, this will be a few seconds into the pull (casters have cast times, mobs need to run to you first, etc.).

For a DK, you generally want to use your cooldowns proactively as opposed to reactively. That is, try to anticipate the incoming damage (which just takes experience), and use the cooldown accordingly. There's a pretty marked difference between "wow, my health just hit 35%--I should probably use a cooldown" and "I know my health has a high chance to hit 35% in about 4s, so I should figure out what I'm going to do about that."

Whether that means you hit the cooldown before or after you take the damage (sometimes it's better, sometimes it's not; depends on the cooldown), the point is that you aren't taken off-guard by that incoming burst, which lets you think more clearly and not do something stupid.

Pretty much this. All of our CDs have pretty short cooldowns, so feel free to be liberal with them. It's best to use a CD and not need it than to not use a CD and need it.
My main tank used to be a prot paladin, but I have tanked a lot on a dk as well. My general philosophy with cooldowns was that short ones (30 seconds to 1 minute) I'd use on cooldown, and basically cycle through them. The cooldown is short enough that it should generally always be up when you want it anyway.

As for longer cooldowns I'd always save them for bigger damage phases or oh !@#$ moments.