Recently, my daughter gave up sucking her thumb, something that I never thought she would/could do. We tried everything-- having the dentist talk to her, limiting her to her room when she sucked her thumb, putting nasty stuff on her thumb.
She liked sucking her thumb. It gave her comfort.
She had no intention of quitting.
One day, after looking through some pictures of her where her teeth were obviously askew from her habit, I tried yet another ploy: the infamous sticker chart. We'd offered a sticker chart before, but she never lasted a single day. This time, however, she was enthusiastic.
She knew just what she wanted her prize to be for a month of thumb-free living-- this atrocious Barbie horse that oozed pink. She only had one day where she did not get a sticker.
Quitting turned out to be pretty easy, but that darn horse had almost nothing to do with it. She's hardly played with it. She just decided she was ready to quit.
It was the same when my son toilet trained. After months and months of presenting big boy undies, bribing him with candy, and pleading, one day he just decided it was time.
And most of the time, this works for us. We introduce a concept we'd like the kids to work on (brushing their teeth twice a day, making their beds, giving up a bad habit) and then try to find the specific motivation to lure them toward their goal. My son is much more led by reward; doing something fun with us or receiving a prize. My daughter has to believe it is her own idea-- bribing her doesn't really work because she is intrinsically not extrinsically motivated. She's much more motivated to do something when the "reward" is a new "big girl" privilege-- like skipping a nap now and then.
By trying to figure out how out kids are motivated, and personalizing our efforts towards their preference, we've had a lot more luck getting them to grow and change.