We started around 9 mths with Meg and she is the most AMAZING signer ever. She is 12 mths now and she knows a TON of signs

. Her first were sleep, milk, and rabbit (her lovey), but she learns them very very fast. DS was a fab signer as well - we started at 12 mths with him and he learnt pretty fast from there.
Starting early is great but just don't get disillusioned if they don't catch on until nearer age 1 when their motor control is better. They will get it and it is a fabulous tool to communicate with.
Watch out for them signing stuff different to what you teach as well - it often won't be perfect but it doesn't matter as long as you both know what it is - and if you recognise it and praise etc it reinforces it far better than trying to teach the correct way iyswim??
I also agree that signs that they are interested in are easier to teach than ones that may be 'useful'. You need to capture their attention with it first.
I swear Megan thinks if she waves her arms about I know what she is saying. She definately knows that waving arms communicates things.