The answer to this question evolves as you grow and change as a writer. My first goal was to finish writing a book. Next was holding a book in my hand. Getting a short story published was also a goal.
Currently, my next level of success will be:
1. Making enough money off each book to pay for the next one (editing, design, marketing, etc)
2. Finishing a trilogy
The first one I kinda completed. I did make enough money from the last book to pay for the next one; life just got in the way so the money's no longer there. And I'm well on the way for goal number 2; I'll be quite happy when I complete that one.
Ultimately, though, my true markers of real success are the following. I think I've shared these before, because I've held onto these goals for a long time. Once I hit these, I will know I have made it:
1. Seeing someone read my book on the bus or subway. At which point I will lean over and whisper, "Not very good, is it?"
2. Finding my book in the discount overstock bin for $0.99 at Chapters/Indigo.
3. Having my book turned into a shitty, low-budget Canadian indy film, a la "Jesus Christ: Vampire Hunter."
The first Wednesday of every month is officially Insecure Writer’s Support Group day. Writers post their thoughts on their blogs, talking about their doubts and the fears they have conquered. It's a chance for writers to commiserate and offer a word of encouragement to each other. Check out the group at http://www.insecurewriterssupportgroup.com/.