I don't recommend the powerhead solution at all... I've never seen it work and it only stresses the animal.
If you can get to it--physically get your hand to it--then here's what I always do and it's worked EVERYtime with no stress or damage to the anemone and it immediately attaches and stays where I want it:
(if you have a clownfish I highly recommend separating them for this because I didn't and it scared the crap outa me everytime that little snot bit me) Take your finger and try to find a part of the foot that isn't firmly attached to the rock. Then use your fingernail to *gently* and *very slowly* get under the foot and slowly work your finger between it and the rock. Take your time and you'll feel it peeling away like a sticker. Once it's off, gently nudge it where you want it and hold it there with a fish net for a minute or two and it should be attached enough to withdraw the net.
I've done this to my little 4" BTA and my gigantic 10" RBTA. It works, you just have to be able to get to it. I've tried all the other methods and had no luck; not to say they don't work, but to each their own, eh?
Good luck~
