A common combination is to give some useful part of the subscription away for free (especially if this has little cost for you) and promote it. Getting traction for a free service is usually easier. Then charge something for more functionality. There's much to be said for high volume, low conversion models. If only 0.5% of your users convert to paid, but your free product is so good that you have a huge community, it's clearly worth it. An example might be AVG. Whatever you think of their virus scanner, the business model is working well.