Most sales people will tell you that you are much more likely to sell an existing customer than a new customer. Why not help this process along by promoting "in" your products? If someone is listening to one of your tapes or watching you on video or reading your book they are obviously interested in what you have to say. Even if they got the product free, you can consider them a customer.
Without being blatent, you can make reference to your other products and services in the course of giving the listener/reader good information. For instance, I have never had a complaint from one of my TeleSeminar attendees because I mention other products and services that I know would be good for them. I don't only mention my stuff either. As long as you are legitimately helping your clients, you will have very little trouble mentioning other products and services. For example, one of the TeleSeminars we did was with the radio genius Joel Roberts. He did a masterful job of telling the TeleSeminar attendees how to be a great guest on the radio. Do you think these attendees minded hearing about our TeleSeminar with Joe Sabah on how to "Sell" on the radio? Of course, not. Do not make people pay for an infomercial, but do let them know about your other products and services that can help them.