One thing that I've noticed really has helped her with her attention is when I say "look at this" I first get her attention and get her looking at me and my face, then make an L at my eye level where she can see it and say "look" having her follow my L to what I want her to look at. Then I'll ask her which she wants. For whatever reason if I say "look at the dog, Violette" she would look at me, and not turn her head to look the direction I wanted her to. With this little technique, it seems to have helped her learn what "look" means.
I also will sometimes notice if she is holding onto something, she won't sign - I'm not sure if she thinks her hand is too busy holding up what she's got it on to sign, but usually I'll move it and let it be free when I want her to sign.
Violette loves popsicles. That was a great motivator. The sign is pretty easy (we use the sign for ice cream), and I could work on "more" and "popsicles" and "please." I'd say if you are working with your child on the signs, try to find a sign that is an easy "win." "Cookie" was another one that worked well to get her motivated.
I did some hand over hand at first, where I would first model the sign, then pick up her hands to get her to do it. I alway make it a game - like it is the most fun thing in the whole world, and I praise her when she gets it right. I also think you need to be pretty loose at first in praising the early attempts at signs, even if they aren't perfect. Communication is the name of the game.
We also will sit in a circle and play games with her sisters and we all sign. It can be with a ball, or with music, or with a toy animal. We always play fun games with the signs for "wait" "go" and "stop." That's fun in almost every situation - particularly on the swing set.
Or I'll quiz her sisters on all the signs they know - and her father too. It was really funny one night. We were playing show me the sign, where I rapid fire ask everyone to show different signs they knew. Violette was right in the game, watching and playing along. Then we played "let's quiz Daddy." He's getting better, but doesn't have nearly the vocabulary that the older two girls and I have. So I was rolling through signs. I got to the sign for "Game." http://tiny.cc/C3Gvz He was totally perplexed, and I happened to look over at Miss Violette and sure enough, she was signing GAME perfectly. Cracked all of us up, because none of us knew she knew the sign but she sure knew she did!
The really cool thing that is happening lately is that she is saying the words as she is signing them. Everything I read told me she would talk when she could talk, and so far, it sure has been the case!