The most obvious way to combat this is to hire an editor. It's not as expensive as you might think, and the end result (putting out a polished product) is worth it. Also, you may want to enlist the help of beta readers. These readers can consist of friends, family members, writer friends, etc., who aren't afraid to offer constructive criticism.
It hurts, I know, but no one ever improved by being told: "Great job. Keep it up."
Still, before outside editors and beta readers are called in, it's important to develop your own editing and proofreading skills.
First, you'll want to make sure you have a solid (updated) grammar handbook. If you aren't sure of a rule, look it up. Apply it. Soon, it will become second nature. I've heard the publishing industry follows grammar rules outlined in the Chicago Manual of Style. (If this has changed, let me know.)
In the meantime, here are a few methods I use when editing my writing.
1. Read through the entire manuscript (ms) once, searching for plot or character inconsistencies. Instead of editing as you go, use MS Word "Track Changes" to flag places that need work. If you can read through your entire ms in one sitting, you're more likely to find those holes or irregularities in your writing.
2. Go line by line, sentence by sentence, making sure each one is phrased the best way possible. When I do this, I bounce around the ms. (It helps me focus on that particular sentence). I just use the highlight tool to keep track of the places that I've covered.
3. Read your work backwards (start with the last sentence and work your way to the beginning). This will help you find sentences that may run on, or comma splices (two independent clauses that are connected with a comma). This also forces you to slow down and read what's really on the screen as opposed to what your mind thinks you've written.
4. Read your ms out loud. This helps with voice issues; you can also determine if your dialogue sounds natural. It's easier to focus on the pacing and flow when you read your own work aloud.
5. I also use the MS Word "Find" feature to check for inconsistencies. I search for characters' names to verify spelling, I search for descriptions (blue eyes) to make sure the character descriptions are consistent, and I search for linking and "being" verbs, to see if any can be eliminated (among other things).
6. Print your ms out. Proofread. Rinse. Repeat.
7. One of the last rounds of proofreading should be someone (or something) else reading the ms back to you. I know the Kindle has a feature that reads your script for you. I downloaded a free program (Speakonia) that lets me copy and paste my text into the box, and reads it back to me. You don't know how many "a's," "the's," or "to's" I've missed in proofreading (even after several rounds), but caught when the computer didn't "read" what I thought was there back to me.
These strategies are all beneficial in their own ways, but it's not up to the writer to pick "one" that works and rely solely on it. I use each of these strategies multiple times. Yes, it takes me longer to edit a novel than it does to write it. It's time-consuming, but WARNING: skip this important part of the writing process at your own risk.
Readers can pick up on grammatical errors and typos in the sample of your work they download. If there are more errors than can be overlooked, you've lost a sale. Thousands of new ebooks are being uploaded every day. Now, more than ever, you want your work to attract attention, to be as polished as possible, to stand out.
If you have any proofreading tips/tricks, feel free to leave them in the comments!