I use the below simple macro and I've never lost aggro:

/castsequence reset=target Icy Touch,Plague Strike,Pestilence,Blood Strike,Death Strike

Blood Strike can (and probably should) be changed to Blood Boil for maximum AoE. So, I pull with this macro (which will use Icy touch), then quickly hit my F1 key which uses "Death and Decay" and drop it right in front of me, then the rest of combat is just me spamming my macro - theres so much AoE damage/threat from DnD plus frost fever and blood plague being spread to all mobs that my shamans can't pull aggro away at all. If I really want tons of extra threat/damage, I just hit my key for unholy blight as often as I want/need to since I always have plenty of runic power.

For people that don't have/want unholy blight, you can actually add:

/cast Rune Strike

to the beginning of the above macro and it will use it whenever its available without breaking the rest of the macro.