I know you're joking, Mrs. Smith, but I do spend time there. I also spent time trying to "fix" my site, but what Google wants is more of a mystery than ever.
Here are some things that seem to be generally agreed upon. Google wants fresh content, and not "thin". The meaning of "thin" has been debated for well over a year, but it seems to boil down to content that's either just cut and pasted from a manufacturer's site, or content that doesn't convey much information.
I have well over a thousand pages of photos and specifications of guns from various manufacturers. The text was cut-and-pasted from the manufacturer sites. My site became an "authority site" to some extent, and Google rewarded it as such. Now that sort of content drags a site down.
I took the Springfield Armory pages and re-wrote the content. I also added a lot of information about M1A's. And I added forms visitors could fill out to offer their reviews of the various SA rifles and handguns. Now that I have probably hundreds of reviews, my rankings for Springfield Armory search terms have gotten better.
The reviews are seen by Google as fresh content. Moreover, Google also is able to see that they're written by visitors, which is a plus. It's like social media. Getting mentioned or linked to on Facebook or Twitter or Stumbleupon carries a lot of weight.
If you're selling things, consider re-writing the text so that it's 100% original. Where I spent a lot of time writing a lot of fresh content for a page (for the M1A, for example), I actually submitted those pages for copyright. Yes, your content is legally copyrighted when you produce it, but if you register it, you can complain to Google and get results if someone copies your content. You can also sue for real money. :)
Links were once important. They still are, but it's now quality over quantity. Paid links are supposed to be frowned upon by Google, but the top-ranking sites are using paid links. You just need to do the links in such a way that they don't seem blatantly paid for.
Whatever you can do to get followers and activity on Facebook or Twitter, do it.
I don't know how much content you have on your site, but you should review it all for originality, "thickness" (the opposite of "thin"), and viewer interest. The Panda update (I consider it a penalty) will drag down an entire site if even some of the pages aren't up to par. So, you either fix those pages or get rid of them. I no-indexed my thin pages, but that didn't do the trick. I'm trying to do a new site that will make more money. If I can't do that, and I wanted to keep my current site, I would go back and re-write every single page to make sure it has meaty content. If there's a page that just can't be made meaty, I'd just get rid of it.
If you're wondering why I haven't already re-written all of my pages to get my rankings back, it's because it would take too long. I've spent an entire day re-writing one or two pages to make them meaty, and there's no guarantee that Google is going to like them.
If you don't already have Google Webmaster Tools, get it. Then look at the pages that Google is reporting having problems such as 404's, 500's, etc. Find the problems and fix them. Sometimes you just can't, such as when other sites are linking to pages in your site that don't exist. I suspect Google can tell that those site owners screwed up.
If possible, where there's a page with a lot of 404 errors, create a page with the URL and put something there that will make Google happy.
Do a search for scraped content. Put the first dozen or so words from the first paragraph of one of your pages in quotes, and see how many sites have copied your content. If you find copied content, you can either try to persuade the site owner to change it (good luck), you can complain to Google about copied content (good luck there, too), or you can re-write the copied pages and then include them in your submission to the Copyright Office. That way, if you find them copied again, you can file a DCMA complaint with Google, which should result in the offending site's copied pages being de-indexed.
Ten years ago I could get a new page on a site to rank on the first page of Google within 30 days. Today? It could take two weeks, two years, or two centuries.
Oh, and as for the "keyword optimization" you referred to, forget about it. The days of keyword stuffing were gone long ago, and keyword percentages mean even less now. Your text should read naturally, with keywords showing up naturally in your sentences and paragraphs. You can get some additional Google traffic by using what might be a common misspelling of one of your keywords, but I wouldn't go too far with that technique, as Google supposedly punishes bad grammar and spelling (although you'd be hard-pressed to prove that by their search results).
