Don't read other people's archived entries if you've just met them in real life. You wouldn't want them to judge you by what sort of ridiculous state you were in a few years ago, would you?
Don't make that bitchy post public and pretend you don't want the subject to see it. It'll feel good for about 30 minutes, and when you realize what a bad idea it was, you'll probably be away from your computer unable to close it for half a day, or they'll have seen it the minute you posted it (you're not the only one who spends too much time on the internet) and will have copied and pasted it to their entire email list. Use your journal for good, or if you must, memes and quizzes, do not use it to perpetuate your drama and then complain about drama in your life for the subsequent three posts.
Witty nicknames are good. Vague references that are so unclear that you yourself won't know what you were talking about in a couple of months are not good.
Make a decision about whether your journal (or even a specific post) is of the "I have an online diary" variety, or the "I have a blog and a readership and wish to be read" variety. If it is the latter, copy-edit the fucker. You know what? Copy-edit it anyway. Small errors are fine, but you should strive to have 99% painlessly readable content.
If you know that your ex, or your so's ex, or your ex's so, or your ex's so's ex, or whoever you feel really insecure about has a livejournal, do yourself a favor and don't read it. Especially when drunk. Or lonely. And if you do, you have no one to blame for what you find but yourself.
If you ignore the above advice and read hir journal anyway, for the love of god don't comment.
Don't make an open post that you wouldn't be okay with your most easily offended computer-literate relative finding.
And most of all:
Don't hate yourself for what you posted 6 months ago now that you're older and wiser.
*NB: Many of these are things I myself have done. Some of them, fortunately, I have not done, but have been the unfortunate witness to.