I've just had one of those weird practice sessions where you change something and it all comes together for a brief while and you're turning the ball at 45 degrees. All I did was relax the grip and give it a flick using the wrist as you've said and not focussing too much on the finger, but then I added a new aspect that I've never done before...... I watched the ball out of the hand focussing on making sure the seam presentation was absolutely perfect and bingo! Ball after ball was turning like Shane Warnes ball of the century off of a newly rolled paddock wicket that is pretty flat. Strangely even though I was looking at my hand I was still able to bowl a good line and length. See my main blog for details. http://www.mpafirsteleven.blogspot.com/
The hope is if I get out there again tomorrow night I'll be able to replicate the same stuff.
Well, I'm no further advanced than hand-to-hand at the moment, but, even then, I find a relaxed grip and big flick gets far more revs with a far more consistent seam angle.