Dont use conflagrate in your castsequence.

It's best to have it on a separate key - then fire it off just before immolate wears off, so you maximise the damage from both. Of course if the mob is going to die soon and is beating on you, just hit the conflag key to finish it off.

(hmmmm - just realised I haven't played my lock since 3.02 so if they have changed the way conflagrate and immolate work together ignore the above).