Author Anne Lamott says in her book "Bird By Bird: Some Instructions on Writing and Life" that you have to understand you're going to write "shitty first drafts." It's as inevitable as the sun rising or spilling something on your shirt right before a big presentation. You will also, because of those horrible first drafts, have to revise your writing. Revision is a necessity and anyone who thinks publishable work happens on the first draft level has a lot to learn about writing or is kidding themselves.
The process of revision can be grouped into at least two categories which will require more than two passes over your manuscript. The two categories are as follows: edit for story and edit for clarity. Edit for story sounds deceptively simple, but what it requires is looking over this thing you have created and deciding what is worthy of staying and what desperately needs to get kicked out. This can include changing characters, filling plot holes, killing off characters, changing scene locations, pushing the story forward in places where it drags, or adding more emotional pull or impact to your characters as well as a slew of other things. Your gut and good writing friends can tell you what's wrong with the story and then your job is to fix it.
After you have edited for story and deleted pages, paragraphs, even continents from your manuscript you should know without a doubt what is staying in. Then you can edit for clarity. This involves the painful task of line editing your work as best introduced by Aimee L. Salter in her Self-Editing series as seek and destroy missions. You search your story for unnecessary words, poor grammar, verb tense inconsistencies, etc., all of which will tighten your work for clarity and make the story more clear and accessible for the reader.
Now, that's a lot of work, you say, and it is. A lot of work which can take days, weeks, months, and even years with the right or wrong author and the right or wrong story. Taking all of this into consideration, wouldn't it just be easier if you wrote a better first draft? It's true, revision makes you realize how much you want to be a better first drafter. If you aren't challenging yourself with revisions, then you can't possibly grow as a writer. My father told me once that life is the opposite of school: you take the test and then you learn the lesson. This also applies to writing since revision work can teach you more about fresh, first draft writing than actually doing the first draft writing can.
For example, if you realize through the revision process that you use the word "just" too much and every time you turn around you've put "just" in the dialogue, the exposition, descriptions, and so on, I guarantee you're going to quit writing "just" so damn much on the next story you write because you're sick of seeing the thing and having to delete or replace it a million times. My problem, as you can see by the beginning of this paragraph, is using run on sentences. My new job, therefore, in first draft writing is to write shorter, precise, more direct sentences.
I promise you if you put on your armor and go into battle with your manuscript to fix both story and clarity you will become a better first draft writer because you will be made aware of what you're doing. Man is programmed to learn from mistakes, so take this opportunity to use the lengthy, tedious, yet rewarding revision process as your teacher for how to conscientiously construct a better first draft.