One of the best things a SWE can do for their career right now is create an online presence and build a personal brand. It increases their reach and potential job network. It gives credibility to their expertise. We all kind of know this intuitively. 

My concern is that what I know best and what I can/want to share with the world is usually what I'm working on for an employer. However, I'm aware my contracts state that whatever I do as part of my job is the company's IP. 

How do people work through this? Do you write blog posts that are just generic enough that it doesn't matter if it came from company IP? Do you only write about your side projects?

I'm asking about blog posts specifically, but this applies to any kind of online content (tweets, videos, libraries, etc). Also, I've made sure the contract doesn't apply to work that I do on my own time, so that stays under my ownership.