Make your songs available on many different formats. While digital formats such as MP3s are extremely popular, don't underestimate the relevance of CDs, or even cassettes. Vinyl is also experiencing a resurgence in popularity. Having your music available on as many formats as possible will create fewer obstacles in letting others hear it.
Distribute your music. If it is available on many formats, make sure that people know about it. Tell others that your music is for sale -- or available for free, if that is your choice.
Meet people actively. Approach them with an open mind. Even if somebody is not a major player in the music industry, they may hold the keys to a very fruitful collaboration, or may be able to reconfigure the industry, so that it is fairer to all musicians.
Play shows often. By playing more shows, you increase your audience and turn new listeners onto your music. If you have your music available for sale at your shows, new fans may end up sharing your music with their friends. In this way, your music can reach more and more people.
Create an online presence for you, or your music. There are many online services that can host your songs and/or represent your musical pursuits. Soundcloud and Bandcamp are great services for hosting your music, and Facebook and Myspace provide great outlets for spreading music-related news.