One prevailing sentiment among writing forums is to hold off submitting until your book represents the best that it can be.
But what does that mean when there is no objective standard by which to measure your book?
Set the damn thing aside for awhile. It is SO easy to be SO excited about something you've just finished, or SO tired of working on it that you just want to start submitting. Give it a little time in the cask to age, and then look at it and see if it's still exciting... or still tiresome. Most likely, you'll notice a few things that need tweaking, and then it'll be ready.
A crit group--a good one--lets you see your manuscript the way your reader will. A crit group will point out that an important bit was unclear in chapter 1, and you'll be able to avoid the confusion that might make an editor give up on your book too soon. A crit group will tell you your spelling isn't terribly consistent. A crit group will nudge you to develop your characters more, or to cut the chapter in which nothing happens. A good crit group saves the editor the large and time-consuming broad-strokes editing that may make the difference between something she can commit to and something that is just too rough.
3. SELF KNOWLEDGE
Is your spelling crap? Do you tend to confuse homophones? If you can't trust yourself to clean the manuscript up, get someone else to do it who can. Lots of little mistakes like that make you seem kind of illiterate, when in fact you may simply be dyslexic. Unless you sometimes lose your hairbrush in your hair, you know that first impressions make a difference. The best writers to work with are the ones who know their weaknesses and their strengths, and work to ameliorate the one as much as they work to showcase the other.
Once you have checked these things off, it is time to remind yourself that:
4. POSITIVITY and ACTION
It's time to try the book out on people and submit it. Maybe it's not PERFECT. So what. Keep working on other things, and keep learning. If you let yourself be the kind of person to fuss over one manuscript for ages without working on anything else or submitting anything, (a) editors are going to hate you, and (b) you're going to hate yourself. Failure to be published is not nearly as soul-crushing as failure to even try.