You might want to check out your local library for Pimsleur. If you're trying to go the audio based route, then Pimsleur, Assimil, and probably JPod 101 would be the way to go.
One way to not forget, and this is something I did with Michel Thomas, was to put 1 sentence for each main concept into Anki with the English sentence for the "question", and then the Japanese "answer" on the other side of the card. You wouldn't want to do this for everything, but just the minimum for each grammar element to keep from forgetting.