We started using signs with my son when he was around 6 months (we started my daughter around the same age 15 years ago). My mom is a retired nurse who worked with profoundly handicapped children for years (who communicated through basic signs) - she figured that, if it helped them communicate, why wouldn't help the babies? We really only used the very basic signs though. I can't really give you any tips - I didn't actively teach him any signs - we just used the ones that he seemed to need to communicate to us. I have to say, though, I highly recommend it during that time where they have a lot to tell you, but they don't have the words yet.
We phased it out, though, once he started talking. We were really only using it as a means for communication, so once he was able to communicate verbally - we allowed that to take over. His frustration level during that pre-communicative stage was low, & I attribute that to the few signs he knew. We also allowed the signs he knew to kind of "grow" in meaning as he attached his own meaning to them. For example, he learned the sign for "more" very fast. He also quickly learned that when he signed "more" he got something, so he started really using it to mean "I want" - he'd point at something & do the "more" sign. It worked, though - he got his meaning across to us!
It doesn't sound to me like you need to change anything that you are doing - try to make sure that the handful of signs you are using are the most useful to your son & stick with them. My kids seemed to catch on best when we introduced one at a time & used them consistently during the learning stage...
Good luck!