For some reason there are some things that are just impossible. By "impossible", I don't mean that it can't be done. I mean that it never gets done. It's absolutely impossible. For example, for those of you that live with roommates (or can remember), it's "impossible" to put the toilet paper roll on the dowel. I find it in so many other places than where it should be, such as on top of the counter, on top of the toilet, or (this is the best one) on top of the empty roll which is still on the dowel. Forget about the debate on which direction the roll should face, I'd be happy with the roll just being on the dowel.
While taking out the empty roll and replacing it with a fresh one isn't exactly rocket science, it just never seems to get done. It doesn't even take more than a couple seconds. So what's the item on your site that becomes the metaphorical toilet paper roll? Are you ignoring some small aspect of your site that just doesn't get done?
There are a few more common items that I've run across that seem just too "impossible" to do, and do regularly.
H1 tags. Don't get lazy with the way that your H1 or H2 tags are being utilized. I've seen these tags in just about every place imaginable (like that pesky toilet roll). For some reason I've seen it on images, on random words inserted within a normal body of text, completely hidden off the page, and others. It's a heading tag, so make it a heading.
Blog. Make one. Add to it regularly. This is admittedly a bit more "impossible" than some of these other suggestions, mostly because the excitement for a blog is highest in the beginning and as you run out of ideas it gets harder. Nevertheless, a blog is helpful as it increases the likelihood of getting links to your site. Try to get in a habit of regularly adding to your blog.
Get rid of old pages (throw away that used up roll!). Especially if you have an older site, you tend to forget about some of the deeper pages that aren't used anymore. Even pages that you mean to take off the site have this weird tendency to stick around in the search engines. These should be deleted or redirected to other pages so that the useless pages aren't stealing any potential SEO goodness from the pages you want to show up. (Hint: Try typing in "site:www.yoursite.com" in Google to see all the pages that are still being indexed)
301 redirects. These can seem tricky, but in the end are actually quite simple. Make sure that your old pages are redirected to new ones. If you have links going to the old pages, you want the benefits to be passed along to your new pages. If you have an .htaccess file, you can get some basic programming information here. If all else fails, get your web developer to do it for you.
There are certainly more things than what I've listed here. In fact, I bet you're thinking about things you've neglected right now. Going back to the toilet paper analogy, the funny thing about it is that it's more of a hassle to not put it on correctly. Out of laziness we don't put it on right the first time, and then we find ourselves fumbling with the roll when we need to use it. Just do yourself a favor. Your site will be better off by taking care of these small SEO items now, rather than later!