When you sign up for any website, for example, Facebook, they will ask you all kinds of personal information.  This is important to think about especially if you have ever read Facebook's terms of service (which are quite invasive).  The truth is, the chances of someone popping up at your door are relatively slim.  What is more likely is someone stealing your password, your birthday and other personal information so they can try to find a way to spend your money. Sometimes this information could be used by someone that knows you. There is something to be said for just having privacy.  Here are some tricks and tips that I follow religiously:

1. When signing up for a website, don't put your real birthday unless you are using a service like Paypal.  Make the year far enough back to put you over 21 but otherwise they don't need to know that information.

2.  Don't put in anything else until you are able to set the privacy of a profile.  Don't use MySpace, because Facebook has much more advanced privacy settings.  I use Facebook as an indirect contacting device that people can find me without getting my email address and I can learn more about them before I give them information.  Only friend people that you know, and set the limited profile settings so that only close family gets your very personal information.  Don't install too many applications, which siphon information off of you to a corporation.  Once you do those things, feel free to put your job information and other stuff.

3.  Don't sign up for websites that make personal information public.  I made the mistake of signing up for OurStory.com, which put my personal information on a public page that was searchable by Google.  This fact did not become apparent to me until AFTER I changed my email address and lost the password. 

4.  Change your email address periodically.  My family gets pretty frustrated with me because I change it about every year.  This means changing your email on every website you're a member of, and all your email lists, but it's worth it.  

5.  Keep two email addresses.  I keep a Gmail and a Yahoo address.  I also no longer use my name as an email address, which is a tool that spammers use to try to find addresses - find something creative and unique.  My Gmail is my personal account, and my Yahoo is my spam account.  Use the spam account for signing up for anything that could lead to spam - contests, forums, websites.  Use the main one for family and professional contacts only.  

6.  If you have a website, you sometimes need to be able to be contacted via an email posted on the site.  If you can, use a PHP form for email, but if you can't, buy a domain name, and then have it redirect to your spam account. Don't create a link to the address or use an @ sign or .com (which spambots can read), but rather write it out like this: myemail  at  domain dot com.

7.  Many websites require a secret question.  These usually come in the form of your mother's maiden name or your first pet.  This is dumb because anyone that knows you closely knows this information and if you ever made an enemy they could just get in.  If you have the option of creating your own question, do so, and make sure it's something only you know.  The alternative is to put an unexpected answer in and memorize it so you remember it.

8.  Recognize phishing.  Ebay, your bank, Paypal, these important financial services that you use that hold some important information, will NEVER send you an email that sends you to a website requiring a password.  The emails they send will always be customer service or informative only.  Phishing is when a group of criminals looking for credit card information will create a website that looks exactly the same, even down to the URL, to get you to put in your password.  Once they have that, they can get in and take over any account you used that password with (especially since most people use the same username and password for everything).

9.  Make a good password, please.  It really does need to contain lowercase and capital letters, and numbers.  The best way to do this and make a password you will still remember is to use 2-3 groups of keys that are close to each other on the keyboard.  You can use it on all your websites, but just make sure you change it every now and then.  

10.  Nigeria is a wonderful place that gets a huge chunk of its economy from scamming people via email.  The first time I got one of these emails I laughed - and then I found out that people really fall for these things.  If a stranger emails you out of the blue offering you money, or threatens you, or even tries to sell you a puppy, and all you have to do is wire transfer a certain amount of money and they will be free to help you, it's a scam.  Take a look at the extended header of the email, and find out the original email address, and then send it to a fraud agency (here's a tutorial on getting headers).

And that's it!  There's really a lot more that you can do but this is a really good start.