If you use any social networking sites (such as Facebook or MySpace), or have a web journal or blog, you should be aware that potential and current employers could be checking up on you.

An example: unfortunate blogger Michael Hanscom got fired from his job at Microsoft in 2003, after posting a picture of some shiny new Mac computers arriving at his place of work. Michael's crime was to mention, in conjunction with the picture, that he works in a nearby department of Microsoft, which his employers claimed could result in a breach of security.

Blogging about your employer might seem a little foolish, but dozens of people do it – and it has, in several cases, resulted in dismissal. Even positive comments can cause you to face problems.

It's not just juicy details about your employer that can get you into trouble, either. Many companies don't consider it below them to keep tabs on all sorts of areas of their employees' (or prospective employees') lives.

Employers variously disapprove of drug use, excessive alcohol consumption, bigoted views (especially racist or sexist views, which could cause problems in the workplace), and inappropriate images, including nudity. Some employers may themselves object to certain behaviours, while others might be concerned about clients looking up the company's employees and finding the same incriminating information.

Silly email addresses also put off potential employers. Free accounts such as Hotmail, Yahoo! and Google Mail are fine (not everyone can be expected to have their own Domain Name, although it can be very wise to register one if you want to look professional), but a username like "sexxxkitten2000" or "always_trollied99" is, unsurprisingly, quite likely to put off prospective bosses.

There are just a few commonsense rules to follow if you don't want to get caught out when blogging or using social networking sites.

- Don't use your real name for a personal blog. If you have a blog about your cat, your family or your personal life in general, use a pseudonym – one that is nothing like your real name.

- Don't blog at work.

- Also avoid revealing your identity on any social networking sites that you don't use for business. If you are on MySpace, there's no need to use your real name if you are putting a picture up; your friends will recognise your face when you add them to your friends list or send them messages.

- Make use of privacy options. Most social networking sites and blog providers have a setting which makes your profile and/or posts friends-only. Even if you're not using your real name in a blog, consider making particularly sensitive posts available only to specified people, such as posts about your job. Your photographs and personal details should always be available only to specified people – not to the general public.

- Don't publish anything incriminating on the Internet that can be traced back to you. "Incriminating" includes anything you wouldn't want your boss, future employer, family or significant other to see.

- Don't use the same pseudonym for everything, in case your employer gets hold of it and can subsequently find you on dating sites, blog networks, social networking sites and message boards.

- Know your rights. You should not be discriminated against because of your race or sexual orientation at work – this is illegal in many parts of the world.

- Have a "sensible" business email address which involves your name or the name of your company, and only use it for business purposes – not for other online activities.

- If your website or blog isn't directly business-related, don't put it on your CV. Potential employers can and do check out websites they find on job applications.

- Just because you posted something three years ago doesn't mean that no one will find it. It may appear on a web search, or a particularly curious employer might spend considerable time browsing back through your blog archive.

- Even if you're not getting up to anything particularly shocking, try to present a reasonably positive face online, just in case. Things like bad spelling and punctuation, or foul language, can really put potential employers off.

By Helena Henderson