With reluctance I reply. It's really tricky. Of course a good voice teacher.
But one of the most important things is .....don't overtax your voice by yelling, shouting, and forcing. Folk singing can ruin your voice if done incorrectly because it has an attitude associated with it that makes it seem like a shout or an "outdoor voice". Projection is not necessarilly volume. It is vocal balance free of tension. Jeannie is right about the Alexander technique.

Problem. You hear blues shouters and high-edged rock singers and bluegrass singers and they may be hurting their voices. One thing, don't throw your head back while you are singing because you are putting strain on the larynx.

And then there is Louis Armstrong, perhaps Dave Van Ronk, Tom Waits and Bob Dylan. Also, Led Zep and other well-known rock singers. Louis managed to keep his voice for a long time but it sounded well....like Louis.

Frank