My rotation

... is only a rotation the first time I cast each spell.  Then it becomes a frantic game of whack-a-mole as I attempt to refresh DoTs as they expire.  Clipping a DoT is a DPS loss, since the last tick is generally the strongest, and failing to reapply a DoT immediately is also a DPS loss (if it isn't on the boss, it isn't doing any damage at all!).

My initial cast sequence looks like this:

1) Shadow Bolt, to apply Shadow's Embrace;

2) Haunt, for the built in +20% to periodic damage;

3) Unstable Affliction;

4) Immolate, which has the same duration as Unstable Affliction and should be chained with it whenever possible;

5) Corruption;

6) Curse of Agony; and

7) Siphon Life

The three instants (Corruption, Curse of Agony and Siphon Life) could be cast in any order, really.  I cast Corruption first because it's glyphed, and more uptime means more Nightfall-esque procs, and Siphon Life last because it's the damage and therefore the lowest priority ... so if I screw up early in the rotation and have to drop Siphon Life to refresh Haunt, I can do so without feeling too bad about the DPS loss.  Some Affliction locks don't bother with Siphon Life at all, but I like the health return. 

My current gear — an eclectic combination of mismatched pieces from reputation, badge rewards, heroics and Naxx 10 and 25 —  is high on haste and low on mana regen, so I'm finding that I have to Lifetap more often than I'm used to.  Siphon Life doesn't entirely mitigate the loss of health, but every little bit helps when you're running Naxx 25 with only five on-spec healers.

I spam Shadow Bolt in between DoT casts, unless the mob is under 25% health.  Then Drain Soul becomes the best DPS filler, due to the 400% increase, and nets a Soulshard (or two, potentially, if it's glyphed).