The cues I've found most effective are:
*Flatten the back
*Contract the abs and bring the pelvis up and back
*Point of deepest penetration
On a related note, many athletes can't get great posterior pelvic tilt because they lack the strength / low-level tension in the abdominals. Another thing to point out is that athlete's with big butts often look like they have poor posterior pelvic tilt when in many cases they actually do not.
