Don't write anything that you wouldn't want your grandmother to read – Remember that anything you write on the Internet is permanent.
Keep your image positive – Eliminate those "fun" pictures of you holding a cup of who knows what. Yes, it may have been water in that disposable red cup, but there's no way to prove it wasn't an alcoholic beverage. If you find negative things about yourself on the Internet create a wave of positive things such as creating a blog to increase positive web "hits" when your name is searched.
Separate your private life from your professional life – Do not link your Facebook or Twitter, etc. account with your LinkedIN account unless both are above-board and professional. Not only will it help you in your job search but it will also give you room to breathe and have your own identity outside of the workplace. Also, choose usernames that aren't related when creating these social networking accounts vs. your professional networking accounts.