This may sound corny, but I believe the most critical factor in marketing yourself successfully is establishing and maintaining a sterling reputation. You can gain expertise through training, put out top-quality marketing materials, and put in long, hard hours, but if you're known as someone who "talks out of both sides of his mouth" or who "doesn't deliver the goods," you're sunk.
Sustained, long-term success depends on a positive image. People need to see you as someone who is honest, trustworthy, diligent, and communicative- in short, someone they can count on.