I persisted with giving the twins dummys ( more for my sanity than anything else) Rianna sucks her thumb, I read a book when she was very young, as I was concerned about the issue of "taking it off her" the twins gave me their dummies in trade for "big girls beds" at 3, but I had been stict with dummies only being for bed, I had a container in their room that if they felt the need for a "comfort" suck they could lie on the bed when they had one, then when they got up, they had to put it back. I found this sooo easy, and now with Riana I too worry about the whole "issue"

Anyway back to this book I read, it said that sucking actually produces a hormone that comforts, therefore why most babies do it in some form or other, and they actually recommended leaving the habit alone until at least 4, when the child is more cerebal than emotional IYKWIM.
They said that "positive" parenting was the only way to stop it, by as already mentioned, not belittling , rewarding, berating, but just gentle reminders,. as apparently it is a totally unconcious habit

I have already tried this with Rianna, and she only sucks if sick or wanting to go to sleep, and once asleep, if it doesnt fall out ( which is the norm) I pull it out, Other than that I make a joke of it, and if she's sucking when not going to sleep we all laugh and say " what are you doing with that thumb in your mouth you cheeky faggot" and she usually laughs and takes it out!! Chris also says to her " let me taste that, it must be good,"and he has a suck of her thumb, again she laughs and takes it out and forgets about it. These are just gentle suggestions Im trying to just "modify" her habit rather than prevent it for the moment, coz I think it will make it easier when she is old enough to stop completely.

Anyway She's not even 2 yet, so I might be just talking utter rubbish, but this is my opinion, from one mum sucker to another!!!! Good luck ( and if you have success let me know)