Compose something every day. It doesn't need to be extravagant or even complete.
Put your first thoughts down on paper, tape, disc or whatever. Not everything you do will be good, but the exercise will yield some bits and pieces that you can later turn into something special.
Listen to music every day. Listen carefully and then apply what you learn to your own work.
Imitate other composers by writing in their style. Imitation is critical to improving your composition skills.
Pick artists you admire, and compose in their style. To imitate without directly copying is harder than it sounds.
Try other styles and forms of composition that you usually ignore. Just because you don't like or aren't comfortable in a particular musical genre doesn't mean you shouldn't give it a whirl.
Choose a simple tune like "Row, Row, Row Your Boat" and try to write multiple versions in various styles like hip-hop, jazz, orchestral, New Age and so on. Without having to worry about the melody, you're free to experiment with structure, chords, countermelodies and so forth.
Record and play your pieces for friends and associates and ask for criticism. Find someone whose opinion you trust, and play your music all the way through. Then ask open-ended, leading questions.
Play the track again and analyze it in detail. Once you get opinions and advice, go back to the drawing board and put all you've learned to work.
Evaluate your past work. Don't let your old music fade away. Dust it off and give it a critical listen. When you let music sit for some time, the warts really stick out. Use this distance from your work to improve your past, present and future music.