Many bloggers just "go with the flow" but I've found that if you plan out your blog before you even start working on it, it saves a lot of hassle later on.
The key thing to remember when planning out your blog, is the topic of your blog, you should always pick a blog topic / niche that you are interested in, a lot of bloggers are creating blogs on topics / niches that they know next to nothing about. The reason being, they've seen content somewhere which says that, these kinds of blogs earn a lot of money, they have high-paying keywords and you'll earn a lot more on this topic then others, this may be true but it spoils the fun of blogging, your not going to enjoy writing about it, so there is no point in writing about it, you should blog for fun, not for money.
The main mistake bloggers make is picking a topic / niche that they do not know a lot about, or do not have a lot to say about, they run out of ideas for articles after just posting one or two. This is also known as bloggers block (Similar to writers block, but blogging).
Once you've found a topic which you think you'll be happy writing about daily, you should brainstorm articles which you'd be able to post on your blog, if you can only brainstorm a few ideas for content on your blog, then really it's not worth even trying to create a blog on that topic. You should also keep this brainstorm for future reference, so if you do encounter bloggers block, you can look at your brainstorm and get some of your old ideas back.
If you've got this far, following the steps in this article, then it sounds like this topic is the right one for you to blog about, but can you honestly see your self blogging about the same topic in a year or two? Do you want to be known as a expert in this topic? A lot of bloggers blogs are great for the first few months, then after that, they just do not want to blog about it any more. You should seriously consider the fact you might not want to blog about, say USB gadgets in a year or so.
Blogs do not start to 'take off' until they have aged at least 6 months, but most of the time after the first year you'll see a great change (for the good) in the traffic / SEO / revenue from your blog , which is why if you are not going to stick to your blog for 6+ months, it's not worth starting it. I'm not saying you can not, some bloggers prefer to create a blog, post content for it for 6 months, then leave it to age. This can work out, but blogs are powered by good updated content, so leaving old-content on their is not going to help a lot, but then you can sell the blog to site flippers.
The last little test before you start working on your blog, do you really have the time for it? Now it may seem when you start off, it only takes a little while to make daily posts on your blog, but you'll find that you have to spend more and more time on it. For those who are thinking about setting up a number of blogs, I suggest you build one blog, set it up, spend a month working on it and then see how it goes, see how long you have to spend on it each day making new posts and what not, then work out if you've got spare time to create another blog.
The first blog you create will always be the hardest, the second will be easier, you know what helps gain traffic, what does not, you know what to do already, most of the time you do not know what your doing a lot of the time with your first blog, so do not be afraid of opening another blog because you think your going to have to spend all the time you did on your first blog, as long as you learn what gains traffic, what's worth doing, what's a waste of time from your first blog, the second blog will be faster.