Unfortunately I am only able to say I am keen to find out others thoughts too
Our numbers are pretty low at the moment, we are improving our activity on Facebook (we have lots of followers) and also increasing the number of posters around the city where our dojo is based.
I've tried to gain interest about our recent trip to Japan from local papers but so far they're not interested which is a shame.
I do find that Facebook is a great tool for keeping people interested between sessions - I setup a dedicated group for our kids sessions a while ago and added parents I can find to this (for both current and not attending kids). We've managed to pick up the kids class to a much better level, attracting people back that had not been for a long time.
I like the idea of a free self defence course, I had even debated a free parents 'taster session' for them to get more of an idea about what we teach their kids and also how the teaching will change as they get older and more capable.