As you know, DD says an awful lot these days (50ish words at last count). I attribute it to the following:
1. She doesn't really walk so she's spent heaps more time learning 'other skills'
2. As you know, we taught her the word 'more' whilst also teaching the sign. We learnt it whilst doing something she enjoys (reading) and after each read of the book I made her sign the word (even if it meant me moving her hands) and at the same time I would say 'more' before proceeding to immediately read it again. She quickly seemed to pick up that doing this sign (and very quickly she learnt to say the word) meant she got what she wanted. I think for her that at this time it 'clicked' that she could get what she wanted by talking. She is now at the point where she will try to repeat everything o say. She will point to something and grunt (her way of asking what it is). I will tell her and ask if she can say it and she will try.
3. We read an awful lot of books. She loves books. She wants to read the same ones over and over. I feel that this has helped her language development an awful lot
4. We point at and give names to most things. When we walk to the park for example we both point out things (tree, bird, car, pole) and say the words.

Like others have said though, she sounds pretty normal. I recently looked up language development and by 18 months 5-20 words is 'normal'