Focus on your content strategy more than anything else. A steady presence with interesting status updates in your audience's newsfeed (think video, links, questions, etc.) will go a long way to keep your subscribers interested and engaged.
Creating custom tabs for your Page can provide additional context to your brand's message, help you segment your audience and provide a place to introduce new marketing campaigns. Tabs can easily be created using the Static FBML application -- use regular HTML to customize the look, but also look for opportunities to provide unique content for your subscribers with FBML tags like <fb:visible-to-connection>. Set your custom page as the tab new visitors see first by changing the Default Landing Tab setting (this currently doesn't apply to Place Pages or merged Pages).
Add even more value through other third-party apps, such as SlideShare, Networked Blogs, Events and PlaceWidget. Premium apps with additional features (photos, video players, offers, e-commerce storefronts, etc.) can be purchased from companies like Involver and North Social.