1. Avoid the 'plain text' option. Probably almost all the mangled posts--the ones with the unprocessed HTML tags and no breaks in the text--are caused by the misuse of the 'plain text' option. Submitting a post as plain text causes all of the tags to pass, so to speak, undigested through the server. But since the webserver serves web pages, none of the carriage returns you enter in a 'plain text' post will appear in the output. I'd almost propose disabling the plain text option, though I can see one use for it: debugging HTML produced by the WYSIWYG editor. But that's the only use I can imagine for the 'plain text' option. Just say no.
   
2. When using the WYSIWYG editor, you can't add HTML tags of your own. Any <b>, <p>, <u> that you type (like I just did), the WYSIWYG editor will, rather than parse, merely echo to the output.
3. When using the WYSIWYG editor, the "HTML Formatted" option is much better than the "Auto Format" option. Both the WYSIWYG editor and the "Auto Format" option add tags to your code; used together, you'll get stuff you didn't intend like four lines of space between paragraphs instead of two.
4. To make a link with the WYSIWYG editor, highlight the text you want to embed the link in, click the 'link' icon, and type/paste the URL ("http://" and all). If you leave off the "http://" part, the editor thinks you are making a relative link, and prepends "http://www.tpmcafe/blahblahblah" to it.
   
5. If you aren't using the WYSIWYG editor, "Auto Format" converts carriage returns into line breaks, "HTML Formatted" does not. If you are in a non-Java environment (like the Lynx browser), or are just used to a plain vanilla textbox a la Haloscan, use the Auto Format option. You can still use HTML tags to embed links and emphasize text, but your single carriage returns  will be converted to <br> tags, and multiple (might take 3) CRs into <p> tags.
6. Set your preferences. Just to your right, under "My Preferences", then "Interface", you can turn the WYSIWYG editor on or off as you wish, and pick the posting mode (HTML/Plain/Auto) that your comments default to.
7. Preview your posts. Last but certainly not least. I hate to sound like your 6th grade teacher, but there's really no excuse for submitting a mangled, misformatted post. That's what the preview function was included to prevent. In fact, it would be a neat enhancement if the server software forced new users to preview for their first 5, 10, 20 posts.
   

What I am going to do here is type a blog post, then enter a bunch of replies to it, because I don't really know exactly how the commenting options work. There are six possible combinations of (plain/html/autoformat) * (wysiwyg on/off), and I suspect that some of those combinations should be avoided, and possibly even disabled