We did baby sign with DS - absolutely wonderful thing. I started on my own at around 6 months (got a book from the library), just with a few basic signs - milk, eat, drink, mummy, daddy. The key is to stick with it - and that's hard at first because it can take a while for them to really get it. It helps if everyone in contact with you and your baby also do it.
I think DS started signing at 8 months - milk. We started doing a class at around 9 months, which helped to reinforce our efforts a bit. By 14-15 months he had around 70 signs and he learnt some more after that but I stopped counting. He didn't really start talking till 18-20 months or so, but he could communicate really, really well, even stringing together signs (and later the odd word) to tell us things from around 10 months.
Some worried that this would stunt his language development, but if that's so, then I'd hate to see what he'd be like if we hadn't done signing. He is a total, complete chatterbox now and stopped all the signs long ago.
I plan to do it again with DD.