This Thing You Call Aggro.

[Concillian]

Basically it's a combination of the right timing on the right target with the right DPS, and while I can't speak from the mage perspective, the rogue perspective is essentially the same.

You can't just go in frontloading damage. You have to meter your damage with the tanks ability to hold aggro.
Be very clear about which target is the correct target, the group "should" discuss attack strategy, though in pickup groups it's kinda rare to do so until the party wipes the first time.

I generally use the gage that if there isn't a sunder on the target, or it isn't below 80%, it's off limits unless headed for the healer.

The reason Arcane missiles is good is that it meters damage out over time. Scorch allows for relatively mana efficient DPS, which is why they are used in instance situations. These allow the tank an opportunity to build hate while you do damage. A fireball straight off will frontload all the damage, while AM meters the damage out over several seconds.

It's not always straightforward how to determine when you can unload. You have to get a feel with how each new tank operates. Hopefully by the time you're done with the 'trash' pulls you have a decent gage of his ability to hold aggro vs. your ability to pull it from him and you can meter your damage appropriately.