On-page optimization goes back to the very beginning of SEO. It has also changed a lot since then. In the old days, on-page SEO was mostly about keyword usage. While you can still use this aspect to create some optimization, it won’t do as much as you would like. This is because on-page SEO has been expanding.
Instead of focusing on keywords, focus on what your users want. You don’t need to repeat keywords your users may be searching for, you just need to have the answers to their questions. The keywords you do use, should be more circled around a theme than they should be trying to exactly match what your users are looking for.
Basically what I’m saying is, if you are still trying to exactly match what your users are looking for, you are as likely to be penalized as you are rewarded. However, if you have content talking about every facit of a topic, users looking for that topic will find you.
It is also important that every bit of information offered on the results page reflect your business and your brand. If the title says the article is about guitars, but the description is about amplifiers – or worse, completely off topic, like kittens – people will be confused and move on.
If you want to see how on-page optimization has gotten here, Almog Ramrajkar has an article on the evolution of the topic. What is important to note is, the old ways are not dead. They still work, but they are no longer all you can do. The few tips I offered here are just the tip of the iceberg to what on-page optimzation offers now.