SEO can be broken down into 2 main categories, on-page optimization, and off-page optimization. On-page optimization involves the changes that are made on your actual site, like through a CMS such as MODx. Changing heading tag, title tags, alt tags, etc., are all examples of on-page optimization. Off-page involves anything and everything that isn't actually done on your site, such as social media, blogs, forums, etc.
Let's start with on-page optimization. This is a lot shorter process than off-page optimization, and a lot less time consuming as well, but requires just as much attention and focus while doing. First lets talk the meta description tag. Your meta description tag gives search engine crawlers a quick hint and preview about the site. It needs to be keyword rich, not stuffed, and relevant to the content of the page and/or site. Stuffed meta descriptions are a big "no-no", and won't get you anywhere.
Real quickly, think about how many images you have on your website... did you know that Google does not crawl images? But there is still a way to get your images crawled for some rankings. You apply something called alt tags. Alt tags go within the img tag, and tell users and/or search engines what the image is. Make sure that your alt tags have a keyword in it, and be descriptive if you can.
Like I mentioned earlier, relevant content for you site needs to be keyword rich, but not stuffed. Ideally, everything from your title tags, heading tags, body tags, alt tags, etc., should contain keywords if possible, and appropriate. But try to avoid using Flash in your site, because it won't be searchable, and search engines don't like it.
Have you ever seen a page that looks like this in the address bar? site/aslodjknaiuq34821q3ur09u2n30h?ASD?!... That's all extra junk on top of a page that can replaced to become a more search engine friendly URL. Simply by using your CMS, or content management system, you can change it so that your URLs that get published, use "friendly" URLs, which will take the page name and end it with .html or .php, whichever you select as your default. And trust me, Google will like you for it. And on the side of URLs, did you know that there's a difference between sites with a www and without a www? Even if it looks like the same site, links going to one could be missing the other, and search engines could rank them differently, so how do you change it so that the search engines recognize them as the same site? Canonical tags, essentially a 301, are added to a site to tell Google and other sites that "site 1" and "site 2" are the same site and all off-page optimization should be considered to both, for they are the same site.
Whoa, this seems like a lot so far, but don't forget, that was only the "easy" part, and less time consuming part. Off-page optimization is where most of your time and efforts are going to be spent. So lets go into subjects about that, and discuss some dos and dont's of each.
An important tool for SEO, are blogs. Blogs are made to spread information about a company, get people familiar with you, as well as keep people updated on what's going on and current deals and specials. Blog sites can be found all across the internet, and the more the merrier. You should try and make a new post every day in your blog, and try to include some links back to your site and specific pages within your site to raise their PR as well as your domain's PR. A lot of people will grab the RSS feed, or follow your blog to keep updated with new posts that you make, so posts should be made "neatly" and professionally. Another part of blogging, are comments. The majority of blog comments are going to be "nofollow" comments, which means that the links and content will not be grabbed by a search engine crawler. This doesn't mean that they aren't useful though. You can gain reputation with an author, and who knows what down the road could bring, possible advertising and other things they could do to help your efforts out. Blog commenting can also bring referral traffic if posted correctly, but avoid being a spammer and post links only when necessary.
Social Media for SEO has exploded in popularity, and is practically necessary for SEO nowadays. Sites such as Facebook, Twitter, Technorati, Squidoo, and Digg are only a few sites which many companies and sites are posting on these days. And once again, they are used for backlinks to your site, if they aren't nofollows, as well as building popularity and grabbing some referral traffic along the way. Profiles can be made on each site with great information about your company, and even include links within your profile back to your site. These sites need to be posted on daily, and kept up with current content about your site, otherwise they're pointless.
Online directories such as dmoz and pegasus, are a great way for some free backlinks. There are hundreds, even thousands of free online directories where you can submit your site. Remember though, to always post only in relevant and appropriate categories within that directory. You can submit to multiple categories as long as your site fits in the description of that category.
Backlinks are links from other sites, that point towards your site. These are important and remember the Pokemon days where you wanted to collect as many holographic cards are possible? Well this is pretty much the case as well, but instead of holographic, redirect that to sites with a high PR. You want as many backlinks from relevant and high PR sites as possible. The more you have, the more search engines will think that your site is well constructed and trustworthy, giving you a higher PR and ranking. The 2 key factors in backlinks though, are links coming from a site with a high PR, and links coming from relevant content. If you're a cloud hosting company, backlinks from a site about monkeys and bananas aren't going to help you out whatsoever.
Forums, forums, and even more forums. Forums can essentially make or break a company. These should be posted on everyday, and make as many posts as possible. First off, after usually your first ten posts, a unique and keyword rich signature should be made with a link or two back to your company. Think of it as a short version of a meta description. Forums are a great way to earn popularity, and earn some massive referral traffic. And if you are on relevant forums to your site and/or company, those people coming from those forums, could be a customer. Always post on forums with a high sense of professionalism, as well as maintaining a friendly and approachable manner. Whenever possible, ads should be placed within the appropriate thread only. When I said that forums can trash your company, here's an example. Many forums are very popular on Google and other search engines, and if someone types something on Google, and a post about your company comes up in a negative manner, you could be in big trouble, because you can't remove that post. Keep in mind, you always catch more bees with honey.
There are many other sites out there to help you with your SEO, such as:
Google Analytics - a great source and tool to track your SEO progress
Google Trends - an amazing tool to decide keywords, and see what is getting searched
Site Yogi - gives a complete breakdown of an entire site
There are plenty of other sites out there that are designed to help you, all you go to do is find them.
While there are still other factors in SEO, these are some of the "core" ones, and all should be done correctly. But don't expect anything to happen overnight, as SEO is a time consuming process, that requires a lot of patience.
A site is like a bride, and SEO is her makeover...leaving Search Engines to be the groom.
SEO is like a garden. First you need to fertilize and prepare the soil. Then fertilize it, and plant your seeds, or foundations. Then as it's growing, trim it and tend to it. Once fully bloomed, any gardener knows, that your job doesn't end there.