ID Theft insurance is a good idea. The average ID theft can cost a person about $30,000 to fix. However, there are a few things folks can do to limit their exposure to ID theft.
Use a password that is a minimum of 8 characters long, contains numbers, letters and symbols. Don't pick obvious things like your last name, "pa$$word" or stuff like that. There are a variety of password managers out there that can be helpful. I use Bento on the Mac.
Don't use the same password for everything. Have a pass wording system. Some people will take the first word in the website's name and attach it to a passphrase. There are many ways to do this, but don't use the same password for all sites.
Your most vulnerable web data is not your bank but your email accounts (and Facebook), from here those who gain access to your information can piece enough together about you (like someone stealing mail from your mailbox) to begin gaining documents they need to forge your ID. Use a separate password for Facebook and email.
Make sure your email service forces the secure socket layer (SSL) protocol this encrypts the data sent between your computer to the end server. Google does this. Facebook can force SSL as well but you have to set that up in your settings. I highly recommend this.
Avoid install software that a) hasn't been scanned by an antivirus program (even on a Mac) or has not been recommended to you by a reliable source. If in doubt, don't install it.
Update your computer's operating system as often as the updates are available. This doesn't mean moving to new versions of the operating system (e.g., XP -> Vista -> Windows 7) but it does mean making sure that your OS is updated with all the latest security updates as they come out. Security updates come out when holes are made public. Not updating makes you doubly vulnerable at that point. This is true for any computer (even Macs).
As a Mac user, there is a particular vulnerability to mac users here. There is a sort of myth out there that Macs can't get viruses or that the operating system can't be compromised. This is not true. They are great computers, but they are nevertheless computers made by human beings. If a mac user thinks they are immune to the sort of threats we're talking about here, they are particularly vulnerable.