Filtering software is great, but it's not the only thing you need. Some kids consider filtering software a challenge to be defeated. And as they get older it can make certain kinds of research a real hassle - such as looking up information on breast cancer.
The most effective thing we ever did was place the computer right in the middle of the house, where everyone spends all their time. That way, even if we're watching TV while one of the kids is on the computer, we can see exactly what they're up to. And they're not likely to want to do anything embarrassing with us looking over their shoulder.
We have access to their email, and they know it.
We also taught our children from a very young age to close the browser immediately if they accidentally stumble onto a porn site. My son must have been six the first time that happened to him, and he did exactly what he was supposed to do - closed the browser and came and told me.
"Don't worry, son. Those were just some bouncing ****ies. You're not going to go blind."