I found these guidelines on a featured Edublog called Huzzah!  They are well written and well thought out, so in the spirit of recycling, I will share them with you here.

Think about your digital privacy:

• Use your first name only to identify yourself.
• Leave off your home email, your street address, phone number, or school name.
• Don’t share specifics of your daily routines that involve time and location (i.e. where & when your soccer practice will be)

Think about the tone of your comment:

• Be polite, friendly, and encouraging.
• Have some humor, but be careful with sarcasm.
• If you disagree, don’t be rude about it; give constructive (helpful) feedback.

Think about the content of your comment:

• Keep your comment on topic and make sense. Say something about the original post.
• Don’t say random stuff or get really silly.
• Be more formal than you would in real life, but not stuffy.
• Avoid texting shortcuts like u for you and l8r for later, and only use one emoticon if you need to.
• Sometimes add a question at the end to keep the conversation going.
• Include your blog url (address) so the blogger knows where to find you.

Think about conventions of your comment:

• Try to fix your spelling mistakes: use Firefox for drafting your  comments.
• Use capitals in the right places: people’s names, places, the beginning of a sentence, and on “I” –no evil i’s. Using all capitals is like yelling.
• Punctuate properly: period at the end of a sentence, space after a period, comma, or end bracket. One “!” will do: you don’t need a string of exclamation marks.
• Remember, you are putting your best self forward, so polish your comments.