A friend told me that I shouldn't use capital letters in my emails. Why not?

You can use capital letters in your emails...just don't use ALL capitals. It's considered rude. It's like yelling at someone.

Look at it this way; if you want to emphasize something in a sentence, you might capitalize the word, right? (example: "I am NOT going to do that" or "I am not going to do THAT") So if you capitalize EVERYTHING, it's like you're trying to emphasize EVERY SINGLE WORD IN THE ENTIRE MESSAGE.

People will actually feel as though you are yelling at them.

What are the pros and cons of changing your email address periodically?

Well, let's look at the pros first:

• Every time you change your email address, you start with a clean slate with all the spammers. If your current email address gets lots of spam, starting fresh may not be a bad idea.
• Internet providers like to get people into their fold by offering reduced "beginner" rates. Of course, once they've got you, you may discover that the price goes up more than you expected. Changing ISPs may get you a better rate.

And the cons?

• You run the risk of losing contacts. No matter how hard you try to let everyone know that you've changed your email address, you're bound to forget someone. Then, somewhere down the road, they'll try to get in touch with you, and won't be able to.
• If your email address is associated with things like internet message board memberships, subscriptions, or automatic credit card payments, you'll have to find ALL of those websites where your old email address is used and update them to the new email address. This can be a real hassle
• Speaking of hassles, let's be honest; it is a hassle for all your friends to keep updating your contact information. Of course, if it's just you, it's not that big a deal, but if EVERYONE starts doing it...

Update 2016: Two significant things have happened since this post was originally written, that would change my answer if I was writing it today.

• There are many more providers of free email addresses than there was when this was written. If you're getting an email address through your internet provider, you're part of a breed that is gradually dying. Getting a free email address through gmail, hotmail, etc, means that changing your ISP is not going to affect your email address.
• More and more people have their own domains. If you have your own domain, you should seriously consider getting an email address for that domain (likely, it came free with the domain; you just need to figure out how to set it up). As long as you've got the domain, you've got the email address!