When you do things worthy of being noted as a part of your brand, you have to document them. I think a blog is a great place to do this, but perhaps you have a page on a website used for this or even just keep a Word document around. However you do it, you need to do it.
When you finish something, volunteering, speaking, publishing an article, leading a group somehow, take a few minutes and jot down some notes. I’d argue that you should clean it up right then, making it something you want to send to a potential interviewer, but if you don’t, at least make the notes. then clean it up when you touch your resume every quarter and have it ready for that next opportunity.