Polish, polish, polish... That's the advice they give you when you're planning to submit a manuscript to anyone. And you should, too. Whether you're planning to show it to an agent, publisher or the drunk guy who keeps playing with your underwear while it hangs on the wash line.
It's good advice, but it's like saying to someone "Cut me a piece of string. I won't tell you how long it's got to be, but you'd better get the right length." I originally began sending Windchaser to agents in the winter of 2009. Of the nine I approached, eight replied. Of those, six were full of praise and encouragement and "please do try other agents" and the usual nonsense auto-printed on pink slips.
I decided not to attempt sending out unsolicited manuscripts to publishers or even to lay myself on the bloodstained altars of anymore literary agents. Instead I opted for the indie route. Eek! To cut a long rambling a little shorter, that meant editing, editing, editing, all on my own.
Thankfully a good friend of mine, and an editor of some experience, stepped into help guide me through the minefield of past perfect tenses, dreaded colon / semi-colon conundrums, the occasional "I before E" cock-up and more than a few plotholes. What neither he, nor I, nor anyone who has read it since was able to do was spot all the typos.
Typos are like muggers waiting down dark alleys to jump out at you and ruin your evening; they're ketchup stains down the front of your best tie that you didn't see because you were too busy enjoying your meal; they are flesh-eating parasites contracted during a wonderfully-romantic sexual episode in a tropical waterhole; they are a pain in the arse.
They remind me of that optical illusion: It dosen't mtater wihch odrer you put wrdos in as lnog as the fnort and bcak lettres are in the rgiht pcale. Your brain just doesn't pick up missing words like "a" and "of", or the accidental use of form instead of from. You can read sentence over and over again and not spot what's wrong with it.
So instead you over-polish. The more you read your own work, the more you spot the odd word or phrase here and there that you don't like, which in turn leads to removing entire sentences, paragraphs, pages, chapters... You alter, snip, cut, prune, and if you're not careful it becomes an obssession - a desperate longing for the perfect manuscript which, in all likelihood, doesn't exist. Seriously, read a couple of commercial large-press titles and count the number of tiny little glitches in them.
The only advice I can offer, right or wrong, is to do what I did: pick yourself a deadline for publication and stick to it. Say you give yourself 6 weeks from finsihing the novel to self-publishing it - if you can't spot a typo in 6 weeks, you ain't never gonna spot it. Then get hold of either a professional editor (often expensive) or a friend with a good editorial eye (rare) and let them give it a once over. You read and polish a chapter, they read it and polish it and then you move on to the next one, accepting that no one is perfect and errors may remain, but you've done your level best.
Otherwise you can end up in a horrendous circle of correcting typos, rewriting and adding new typos, correcting those typos, rewriting and adding ne.... You get the picture. Far more important than the aesthetics are things like character, plot and atmosphere. Nail those, if you can, and people will forgive the occasional faux pas.