Many independent developers make Android or iOS apps but they do not have a budget for promoting the new app, or the budget is very small. This is a problem because Google Play and Apps Store have so many apps that make very hard for a new entry to rank well.
Every developer have access to a blog or can make one. So one must step after a app release is to write a post there. The one post will help promoting but is just one page. If you have a blog with alot more articles and a good traffic it is a must to promote your new app on the sidebar or in the footer of any page.