Opportunity

Create an environment that increases the frequency your child will have to practise signing.  When you are working on a sign for a particular item, do not allow him to have open-access to that item.  For example, if you are teaching the sign for "book" make sure that your child's favourite books are within sight but out of reach so that he will need to interact with you in order to obtain the book.  Although this takes more time, this method will increase the frequency that you can teach the sign for "book", increases the number of times your child can practise the sign for "book", and therefore increases the liklihood that he will be signing "book" independently in a shorter period of time.