DD is 15 months and we have been signing with her since about 9 months. RIght around a 13-14 months her signing skills took off and she now has about 15 signs and uses them all correctly. Even "read" which seems kind of abstract to me. SOme of the signs I did over and over again for months (like "cat") and it took her months to get it, but then "read", I only did it maybe twice and she could use it correctly.
I think what has worked for us, is just Dh and I using the signs in context and not trying to teach them to her directly. ALso, when she made an attempt at the sign we gave encouragement because her fine and gross motor skills are not what ours are so I did not expect her to be able to do them exactly (for example, horse and cow are, for her the same sign, as is book and open...but based on the context of the situation it is pretty easy to figure out what she wants).
ANd as her skills have developed her signs have become more correct.
ALso, we have introduced new ones as needed. It is also a learning process for us. We didn't do this before and we have to look us the sign and remember to use it, etc.
As far as doing the signs all at once, DD has done that in the middle of the night in her sleep...just processing the day I guess.
And also sometimes she will babble at us and her hands are always moving. I don't know if that is normal or just because she she has learned to communicate this way and thinks that is what all talking involves?
Praise and persistence is what has worked for us and it really is a lot of fun for all of us.