I have been reading a lot of fashion blogs lately (my current following is about 20 fashion blogs I follow.) I have decided the best way to make a fashion blog successful is a couple of things.
1) Be on trend
Fashion is about trends, trends come and go sometimes within months, a lot of times by the time you hear of a trend it's already dying. That's why being on time is useful for something as ever changing as fashion, good blogs are the ones that can point out the new trends and show their viewers how to pull off the new look. Blogs are also a great way to spread the news of new lines, new collaborations or generally what is going on in the fashion world, and as there are so many blogs somebody is bound to be talking about what you want to know.
2) Have lots to say
What I mean by this is you talk about a topic in more than one way. For example my best friend has a make up blog, she doesn't just talk about make up but she does tuturials of how to do her looks, does reviews, and shows off her new make up. With so much diversity there's always something new and interesting. The best fashion blogs I've seen usually talk about more than one thing, pictures of themselves in an outfit, reviews on fashion runways, fashion photos, new line information, contests, etc. This makes the blog a surprise and a delight to see updated.
If you want to make a well known blog then it's going to take time. You have to think of things to talk about, to take pictures or to research trends, or pick out an outfit, or do whatever it is that you are going to do for your new blog post. All of these things are going to take time to do, if you want them done well. As most well known blogs post at least very often this can equal to a lot of work, effort and time. This is why some people who don't want to put in that much time (like me) have blogs that are just for fun that aren't updated very often and a bit random with what's posted.
4) Thick Skin
So assuming you make a fashion blog and it gets really popular, it's trendy, it's funny, it has a lot to say and it gets updated all the time with lots of great stuff, and it's well known, well that's when the less fun people join the party. These are the people for whatever reason, maybe they dislike people being popular while their own blog goes unnoticed, maybe they hate fashion and think it's a waste of time, maybe they just like to annoy other people or even maybe it's a well meant criticism, somebody is going to say mean things. After all this time and hard work that was put into the blog, somebody is going to come in and anonymously be a jerk, and there's more or less nothing that can stop it. (Without going to drastic means and taking away commenting.) It's annoying and hurtful but it unfortunately and unfairly comes with being well known and writing publicly.
However sometimes even after doing all these things I said a blog doesn't become famous, or a blog that seems to have no time spend at it at all is unbelievingly famous. A lot of these things comes down to dumb luck. Sometimes things become popular because that's life and that's how it rolls.