Be as anonymous as possible. Avoid postings that could enable a stranger to locate you. That includes your last name, the name of your school, sports teams, the towns you live in, and where you hang out.

Protect your info. Check to see if your service has a "friends" list that allows you to control that can visit your profile or blog. If so, allow only people you trust. If you don't use privacy preferences, anyone can see your info, including people with bad intentions.

Avoid in-person meetings. Don't get together with someone you "meet" through a blog unless you are certain of his or her actual identity. Although it's still not risk-free, if you do meet the person, arrange the meeting in a public place and bring some friends along.

Photos: Think before posting. What's uploaded to the Net can be downloaded by anyone and passed around or posted online pretty much forever. Avoid posting photos that allow people to identify you (for example, when they're searching for your high school), especially sexually suggestive images. Before uploading a photo, think about how you'd feel if a parent/grandparent, college admissions counselor, or future employer saw it. 

Check comments regularly. If you allow them on your profile or blog, check them often. Don't respond to mean or embarrassing comments.  Delete them and, if possible, block offensive people from commenting further.