Sing songs you like in the style of their original singer. Pick several songs from different singers with unique or unusual voices. Include singers with strong, studied voices and other singers with rougher, less-polished singing styles. Pay attention to what sounds good when you do it.
Try singing a single song in the voices of several different singers. You can pick a song you wrote, or just one you like. Comparing different styles in the same song will allow you to get a better feel for the strengths and weaknesses of each style.
Change your register. Sing a song higher or lower than you normally would sing it and see how it affects your voice.
Make yourself feel very happy by thinking about something you love or enjoy. Then, sing a song. Now try doing it feeling sad. Go through as many different emotions as you can.
Pick a song with a special personal significance to you. Now, imagine singing it to someone you really love. Now, think about singing it to someone you can't stand.
Combine everything you liked from the above steps into a new singing style. If you like your singing best while imitating Townes Van Zandt, singing below your normal range and feeling lonely, that's the way to go.
Refine your singing style. Try to move away from imitating a particular person and make the style feel more like you. Developing your own voice is an ongoing process.