Directing flow at it until it starts moving can work, rubbing an ice cube around it's foot coupled with massaging the area can also work, I find the quickest and least labor intensive is hanging the rock upside down in the air above a bucket until the nem drops. You can periodically squirt it with water to keep it wet. Whatever way you choose, prepare to exercise patience. It can take 30+ minutes for any method. The true easiest way is catching it when it's moving on it's own. If it's on the glass, you can work a credit card slowly under the foot and pry it up that way.
The best way, in terms of the nem’s health, is to chip the rock away, slowly and carefully exposing the foot. Granted, it’s not the preferred method because it requires removing the rock and essentially destroying it, but it’s by far the best method to avoid stressing the nem.
I agree with D-Nak. When my Magnifica split, I took the rock, an old dead tank raised Monti Cap and just chisel a piece of coral the anemone attaches to and gave it to my friend. Minimal stress to the anemone.