This is for relatively new Affliction Warlocks such as myself. I read somewhere in these forums that the ticks shown on the Quartz cast bar (and possibly other bar addons) are not accurate with the actual ticks of Malefic Grasp and Drain Soul. I did this little fix and it increased my dps considerably:
Get the Drainsouler addon
Set up audio cues on the ticks
Decrease the size as much as possible (is there an audio-only option?)
Disable ticks being shown in Quartz.
Doing this helps not to clip drains or let them run too long, especial in execute. YMMV but worked wonders for me.
Also, I can't find where I recently read this but would like credit them with the info. Thank you, sir!
Drainsouler: *edit* I can't post a link to the addon, but it isn't hard to find on Curse.