Everyone experiences burnout eventually. As a blogger, I experience burnout whenever I constrain myself to a tight little circle of subjects. For example, my gaming blog only kept up as long as it did because I wasn't focused on just games, but on game design, running and playing games, and general game theory. That's a lot of ground to cover even if it all falls under 'games.'
In writing, I've always experienced burnout eventually, since blogging about writing is a very solitary thing. Every person who writes about writing puts out information only from their point of view, a highly draining exercise for an introvert (I'm generally introverted, don't let my blog or twitter presence fool you!).
So how do you overcome that problem?
First, don't get tied down to a single concept of what you're talking about! Seriously, if you start with writing, get more specific. Are you talking about copywriting? Blogging? Grammar? Make sure it has enough depth to talk about for a long while!
Then, spread out. Pick some related topics, things that tie into the core of your content, but are interesting and give you room to explore. A number of people I know who blog about writing also blog about movies (As an audience member). This is a great relationship of topics since a stronger understanding of the one helps the other!
The last tip only works if you're blogging for branding purposes (Meaning your blog is about you, not about a topic), don't worry if you change up to other things that interest you once in a while!