There are people that consider Tenkar's Tavern a popular blog - and for the corner of the RPG hobby that is the OSR, it is. I'm pretty happy with how things have worked out and I'm happy to share the secrets of its success.
The first secret is - there are no secrets.
If you read this blog on a regular basis, you know it is probably popular because of the passion I have for the subject matter. I love RPGs and I fucking love the OSR. If you enjoy writing your blog's content, there is a good chance your readers will enjoy it too. If posting is "work", that will be obvious to your readers too. Blog about something you have passion for and you've already passed the first and largest hurdle.
Engage your readers. Engage other bloggers on their blogs. If you enjoy reading forums, engage the readers there. Engage, engage, engage.
Establish a posting frequency. I'd suggest at least once a week, but whatever it is, try to stick to it. Multiple times a week is even better. I'm not suggesting multiple times a day like I tend to do, because the trick to that isn't just "blogging in the moments between", it also requires a certain type of controlled ADD and perpetual multitasking. I blame it on the chemo from a few years back - I used to read a book a week on average, now I'm maybe a book a year. I am still a voracious reader, but now I look for it in smaller pieces.
Be yourself. That may not be the person you are in everyday life or it may be, but it needs to be a true aspect of you. Nothing is worse than a blogger putting on a false show. It's hard to maintain and sooner or later, the facade will fail.
Do NOT expect to get rich off of blogging, especially in a niche the size of the OSR, or RPGs in general. Heck, don't expect to make any money doing it - this way, if you do make some nickels and dimes, you'll be thrilled as opposed to disappointed.
My personal observations have shown me that Adsense and Amazon are just not worth the hassle for the minimal income they will generate. I have found that referral sales from OBS / RPGNow have raised sufficient funds to allow me to run contests and give away gift credit and the like to my readers. A win / win if you ask me.
Most of all?
Do it. Start blogging. The only failed blog is the one that never was. Even a blog like Grognardia, long since silent, has a voice that still resonates across the internet.