On-Site Optimization is the SEO tweaking you'll do on your blog, as opposed to the work you do outside of it. This part of blog SEO is extremely important, but much easier to do than link building, which is usually a long term, never ending process that blooms over time.

On-site Optimization on the other hand, can be done in a as little as a few days, depending on how much content you have. To keep the principles of blog SEO short and sweet, here's a checklist I would use to make sure my blog's Onsite Optimization is up to par:
Meta Title Tags: Does EVERY page and post have a strong Meta Title, containing the keywords you're focusing on for that specific page or post? Does it compel the viewer to click through to read the article?
Keyword Focus and Optimization: Have you been using Google Keywords to find the best keywords to focus each post on? Are those keywords peppered throughout the post in question in a natural and readable manner?
Tag Relevancy: Are your tags relevant to your content, with the most important keywords listed first?
URL Optimization: Are your url's relevant to the content, and do they contain your main keyword(s)?
Effective Titles: Are your titles engaging? Do they sum up the main value of the content? Do they contain the keyword(s) you're focusing on?
Image Optimizing: Do your images have a description/Alt. description containing your keyword(s)? Do they link to another relevant post? (people love to click on images for some reason)