You give them something that they will pay for. This sounds obvious, and it is, but it’s easy to miss the obvious.
Subscription Best Practices suggests:
Lock up the best parts of a site and you’ll get more subscriptions. How many more depends on the site, the readership and the title. Fortune, with its archive of definitive [...]

NevOn reports that BMW is launching a series of podcasts during the Frankfurt Motor Show, and says:
BMW certainly have leaped into the podcasting arena in a big way with this podcast series. From listening to a couple of the podcasts it’s clear that BMW have carefully thought through and planned their approach to communication with [...]

Every company has a number of audiences: prospective customers, current customers, suppliers, affiliates and associated companies, the media, and many more.
RSS lets you narrow-cast, so make a list of your major target audiences, then create an RSS feed for each of those audiences, and promote the feeds. Each audience will receive only the information [...]