As long as we routinely use dozens of services that rely on "memorable" data to authenticate us, this is as inevitable as people who try to use different passwords for everything writing them down.
Password stores are one possible improvement, but most people don't know enough to use one, and they are probably far too fiddly for most people anyway. And of course, ultimately you're still talking about using a single set of credentials to authorise everything in that case, it's just a different target (which if ever compromised will undermine your entire identity).
Multi-factor authentication is a much better solution, but the technology to make it ubiquitous in a way that is neither excessively expensive nor creepy on privacy grounds isn't there yet.
There are some problems in security that we know how to solve, at least to the extent that no-one has any idea how to crack them directly today and the effort to brute force them is effectively infinite. I'm really hoping that one of these days, the combination of mobile technology and the Internet will provide us with an easily portable device that can integrate with everything and render obsolete the current mess of hundreds of on-line identities, "memorable data" to authenticate for every financial service I use, etc.
