Despite reading and reviewing my manuscripts, those pesky errors always slip through. Most will be so subtle that they often go unnoticed by the reader caught up in the story and not so much the words. I use several methods now to try to keep my books as clean as possible. The first is a product called WhiteSmoke. It allows you to pull in each paragraph of your manuscript where it notes errors and gives recommendations of sentence structure (such as using the same word twice or more in the paragraph). I found it is about 50% accurate in catching errors, but it helps even though the product is buggy. Lord help you if you make a change while in Whitesmoke that it didn't highlight, for it will delete an entire paragraph out of your manuscript if you are not careful, and I have on a couple occasions had it reinsert a paragraph with every space removed - not cool! I've learned to make sure my document is saved prior to pulling a paragraph into WhiteSmoke in case I have to close and reopen my manuscript to get my paragraph back, rather than hitting the reverse changes icon a whole bunch of times. I've tried other products but none of them really does the job well.
I also have a friend by the name of Pete who reads my books and sends me a list of needed changes. He has been great to work with and does wonders on finding the errors Whitesmoke misses. His price? A copy of each printed book - a steal of a deal...