There's quite a few ways, here are a few that we're using for our new app (www.yobiz.com):
* Affiliate Networks - Affiliates who target your markets can really drive qualified traffic.
* Blog posts - good, high quality blog posts on topics relevant to your business can drive good traffic.
* Email Newsletter - Market either to established email newsletters or generate a list from your existing customers
* Facebook Ads
* B2B Forums - Participate in the discussions but be cautious about self-promotion
* Get featured in an article - Spend time cultivating press relationships in order to get featured in press.
* LinkedIn Posts
* Local meetups - Attend local meetups in relevant industries and network, network, network
* Mobile Apps - Create a mobile app that drives subscriptions.
* Press releases - really good for SEO and not much else.
* SEO - good SEO will really help make your app visible.
* Social bookmarking: Digg, Reddit, delicious, etc. Activity aiming to get website posted onto mass-sharing networks.
* Attend conferences & trade shows
* Tweet - be part of the conversation and generate a following with occasional self-referential tweets.
* Video - Video on relevant topics with good quality can really drive traffic.
Hope that helps!