This spell is the quintessential cornerstone of the Destruction tree. It increases the damage of your Incinerate spell by a substantial margin, and is one of the spells that gives you access to casting Conflagrate, and the Fire and Brimstone talent allows it to increase the damage of Chaos Bolt, as well as further increase the damage of Incinerate.
The Destruction talent spell Conflagrate needs an active Immolate DoT (or Shadowflame DoT) on the target in order to work. Once cast, the spell will consume the effect, thus it's advantageous to wait until Immolate has almost finished dealing all its damage before casting Conflagrate.
You can get around this side effect by using Glyph of Conflagrate, available at level 40.
Improved Immolate will increase the periodic damage of your Immolate spell by 30% at max rank.
Bane will lower the cast time of Immolate by up to .5 seconds at max rank. Talents like Emberstorm at max rank will increase the amount of fire spell damage that applies to the spell by 10%.
When tanking mobs with your pet (either Voidwalker or Felguard), save casting Immolate until the end of your DoT rotation. Not only does it have the shortest duration, but as it deals initial damage, it generates the most threat and could possibly pull the mob to you.
However, there is an exception to the above tactic. If the Warlock is soloing, and, the tactic is that of a Warlock Tankand the Warlock is expecting a long fight, casting Immolate to start the fight may be beneficial for two reasons. First, the Warlock avoids potential interruption of the 2 sec. cast time when offering an opening Immolate. Secondly, if the fight is expected to be in the 25-35 sec. range, a second Immolate can be cast, maximizing the likelihood of conducting a mana-efficient battle (2.12 dmg. per mana at max rank)
Patch changes
Patch 3.3.3 (2010-03-23): The damage-over-time component of this spell can now produce critical strikes.
Hotfix (2009-04-15): "We have hotfixed the damage of the Warlock spell Conflagrate from 100% to 70% of Immolate's total periodic damage instantly. The tooltip may still show 100% until a future update though."