Francis Bacon is credited with saying, "Knowledge is power." No where else is that more true than in navigating the internet.
As with parenting and driving, the more about which you're aware and know, the greater your chance for avoiding dangers and being successful. You have to know what dangers exist, you have to have skills (a type of knowledge) to use the tools and techniques in order to maximize their power, and you have to know how to deal with problems.
The focus of defensive browsing is less about avoiding problems and more about maximizing your online experience. Maximizing your online experience involves minimizing the amount of time you need to spend rebuilding your computer after a virus or chasing down faulty credit card charges, or cleaning up spam. If you know how to avoid these problems, you indeed will be able to powerfully leverage the internet and all it has to offer.
In short, browsing defensively is a mindset that translate into behaviors. Browsing defensively involves three main components:
A defensive browser is aware of the potential of the internet without being fearful of the dangers.
Someone who browses defensively is aware of where potential danger may lay, is aware of what methods and tools exist to avoid those dangers, and is aware of when those measures have failed and what to do about it.
Someone who browses defensively doesn't approach the internet with the attitude, "I know there are dangers but I'm going to rely on the safety and security tools in my browser and pay no attention to what might happen." Rather, awareness itself is tool the defensive browser uses to maximize her time on the world wide web.
As with anything of importance, getting good at using the internet takes skill and developing skill takes time and practice. However, browsing defensively is less like learning to fix a car and much more like learning to drive a car.
For most of us, we're much better drivers now than we were when we hit the road for the very first time alone. Most of the skills we formed didn't come from formal education or from being mentored. It came from taking some basic instruction and applying it in real world scenarios. Over time, we got a "feel" for the road and an "intuition" about breaking distances and "safety bubbles" and how fast to take a corner etc.
The same is true with browsing the internet. Browsing defensively will be a skill that you'll acquire by applying simple ideas to your activity on the internet. Over time, you'll find yourself getting better and better at detecting the dangers and avoiding problems.
Once you've discovered a potential risk and gained the skills to deal with it, you have to be able to adapt to the situation and make adjustments to deal with the risk. For example, if you believe you have a virus on your computer but your virus scanner doesn't pick it up, do you have other ways of validating your concern? What if your system was attacked and you no longer can get on the internet? Do you have a plan for dealing with the threat offline (e.g. have other virus and malware scanners on CD)?
So there you have them: Pardi's Three 'A's of Internet Safety Rules.