Before I give my answer (Jim didn't ask me but I love blogging so I will answer it anyway) I'd like to highlight two answers I liked best.
Stephanie Halligan, The Empowered Dollar:
The one thing I would have told myself when I first started blogging is to write from my gut. My blog The Empowered Dollar was originally dedicated to helping moms teach their kids about money… and I’m a 26-year-old with no kids. At the time, I thought it was a profitable niche. But I absolutely hated writing blog posts because it didn’t come naturally and it wasn’t anything that I wanted to write. When I finally changed directions and started writing what came naturally, everything changed. I love everything that I write now, and I’ve gotten more readers because of it, too.
She's dead on. If you're forcing the subject you'll never love it. If you never love it you'll never be good at it. If you're not good at it no one will like it. Love what you write about and it will show through to your readers.
Andrea Green, The Greenbacks Gal:
You will be stupid in the beginning. You will make mistakes. [...] But course corrections are all part of the game. No amount of effort you put forth is ever bad.
Knowing how to blog successfully* involves a lot of things. It involves becoming a decent writer, coming up with ideas to write about, having the motivation to hit publish, developing it into a business, learning about blog software, ad networks, sponsorships, search engines, etc. etc. etc. All of this takes time and trial and error. However, posts like Jim's can help.
My answer; Make your blog your business. Don't write casually or when you feel like it. Don't write in your pajamas late at night. Don't just "remember" to write. Schedule to write. Make your blog your business. Work at it as hard or harder than you would a "real job". Learn your craft, learn your business, do research, read. Wake up in the morning, shower, shave, brush your teeth, and get to work.
Anytime I've seen people approach blogging as a hobby on the side has never really made it until they took it very, very seriously and attacked it like any other business.