You could follow the advice of many commenters here and increase prices. I recommend you increase prices on new customers, but maintain similar prices for current customers, since they've been early adopters of your service. The danger of suddenly raising prices on existing customers (or radically changing the service) is that you might lose them (see Netflix).
This has probably been said before (and/or above), but perhaps each article should be tagged and users should subscribe to only those tags of interest to them. In addition, a social component to HN would be useful -- for instance, allow me to recommend a story for someone or for stories of interest to my friends to be ranked above the norm. Our friends make great filters!