Begin promoting early: don't wait until the release date to start building excitement around your project. Begin posting to your various social profiles the moment you begin conceptualizing your next single, EP or album.
Guise it as a covert operation: this is an optional promotional tactic, but it has worked out pretty well for some businesses. Name your project "Project Penguin" or something ambiguous, and begin to release details a little bit at time. Don't let on that you're working on a new music project right away. This will keep your followers engaged.
Share a little bit at a time: you don't have to share everything as it is completed. You can share little fragments of your writing, rehearsing and recording process to stimulate curiosity.
Share daily: avoid overwhelm by sharing on your social streams on a daily basis. Otherwise, you could wind up with a huge back log of digital media that has outlived its usefulness.
Show the progression of a particular track: from demo to scratch to principle recording, there might be a track that evolves quite a bit over time. Share the progression of the track with your fans.
Market on your social networks: in addition to giving progress reports on your website or blog, post your updates to various social networks as well. Use sites like Facebook, Twitter, Tumblr, Instagram and Pinterest to reach different audiences. Any tool you can use to cross-post will help you expedite this process.
Accept pre-orders: once you have a track or two in its completed form, it's a good time to begin taking pre-orders. BandCamp has some features that you can use for this purpose, and you could also take payments on your website or in-person. Incentivize the pre-orders by reducing the price or by throwing in extras like buttons, stickers and magnets.
Host a Q&A session: once you have a pretty good idea of what your album is going to look and sound like, you could host a question-and-answer session on Google Hangouts for your fans. You could film the entire session, edit it, and put it up on YouTube later too.
Record daily or weekly vlogs: keep your fans updated with your progress with vlogs. Talk about some of the challenges that you are encountering into as well as what you're excited about. You could take this opportunity to play demos or acoustic versions of your upcoming tracks too.
Film a music video: a music video is a powerful way to promote your next single. It may cost a little bit of money, but the resulting attention your music gets will make it worthwhile.