1:
Don't assume everyone follows your blog obsessively
Just because you mentioned your friend Doreen in September, it does not mean that people will know who you're referring to in May.
2:
Don't talk about petty arguments and annoyances
Unless your blog is about annoying things, and even then, chances are that no one will care if you can't find your glasses or the postman was late.
3:
Don't refer to something without referring to it
I'm talking about: “OMSquee! I can't believe that happened. I saw it on the news and couldn't believe it!” That won't mean much a few days later, never mind a few months. The same applies to “WOW! Today's episode of Heroes was THE BEST EVER!”. Why was is good? Why should readers care?
4:
Don't use private jokes
You may think that all your loyal readers hang on your every word and will understand, but most likely it's irrelevant to most people. And it looks arrogant. If your blog is supposed to be there to promote yourself it doesn't work if you keep putting asides to friends. People won't be impressed that you have friends.
5:
Don't get upset over negative comments
Most people get negative comments. Learn to handle these. Do not flame their blogs in return and don't keep referring to “that stupid MissPostPerson who doesn't agree with me”. Why draw attention to the fact someone disagrees with you?
6:
Don't try to be the next X
X is already doing that. You will be one of a million X copyists. It's not flattering to be written off that lightly.
7:
Don't go on about the same thing all the time
Unless your blog is on a specific topic (horses or going to gigs or internet…) then don't keep going on about one thing. It's fine on a personal blog just friends see, after all they're used to you, but random members of the public probably won't care about your obsession. If you always talk about a particular friend or your tutor or the price of socks no matter how insightful what you have to say is, it will seem repetitive.
8:
Don't try to drum up publicity negatively
You've heard “there's no such thing as bad publicity”? Wrong! If you're known as the person who keeps flaming everyone or tries oh so hard to be controversial then you'll never escape that. If someone does find their way to your blog their views will be colored by your actions.
9:
Don't try to make your blog pretty
So you use fifteen different colors and blinkies for your background? Now no one can read what you have to say. And half your readers have migraines.
10:
Don't assume your blog will make you instantly famous
Let's face it, there are a lot of blogs out there. A handful of people have become well known because of them. If your blog is mediocre it's one of many. If it's good it's still one of many. If you're only blogging for fame and fortune you'll get bored pretty quickly. Blog because you enjoy it, and treat anything else as a bonus.
It looks fun!