I was thinking about this the other day, and thought it would be cool to court a few opinions from the pros on this sub about what they wish they could tell their younger selves. My picks:

1. Do your homework. Coming into the first rehearsal with your music at least well-learned if not memorized is amazing. It impresses everybody, and makes the process so much more fun for you because you can focus on playing with it rather than grinding out notes.
2. Practice your fundamentals. Get into the groove of practicing fundamentals like sight-singing and rhythm (I had a comment about a great exercise I posted recently) frequently. You'll be surprised how little effort it takes to massively improve your skills, and those improvements will be tangible in your singing.
3. Write songs! It feeds your soul, and being a great songwriter is the best way I know of to actually make money singing pop music. Every great band has a great songwriter at it's core, sometimes multiple great songwriters.
4. Find a great teacher who you click with and whose skills and career matches your goals. Michael Jackson took lessons until the day he died. If it was good enough for MJ, it's good enough for me. =)
5. Finally, develop a skillset that allows you to work a flexible schedule. A lot of singers choose to teach, but it doesn't necessarily need to be music related. I taught myself web development about 6 years ago and it proved to be one of the smartest decisions I've ever made. My schedule is no longer a slave to day jobs and my singing career has benefited enormously from it.

Anyone else have anything else to add??