1) Trusted friends who you can trust to critique your writing.
2) A trusted individual or individuals who can help with your initial edits.
3) Drive.
Having a trusted individual, or group of individuals to critique your work is essential, and while they don't have to share your interest in a topic (remember, Tolkien hated allegory in all its forms, yet he still critiqued Lewis' works such as Narnia) they will go through your work with an eye not necessarily to subject, but to such things as flow, grammar, timelines, continuity etc. The inklings would gather over a pint, those who smoked would smoke and they would have readings of their latest works, then they would critique. These men trusted each other, and with good reason, they had become friends and while doing so, gained that trust. Now the Inklings did more than that (Tolkien taught some of them old languages) but at its core, the Inklings were about literature.
Someone who can do a preliminary edit for you, before sending it to your agent's editor or the publisher's editor. They will check some of the same things as those who critique your work, but with more of a focus on spelling, grammar and other such details. They will also point out such things as continuity errors and so on. The editor is generally more involved in the technical side of your writing.
Drive. Don't stop, have thick skin.
Oh for a group like the Inklings.
