"If I have to change it every XX days, I tend to pick very easy to remember passwords, and just change a digit at the end of it every time. As a result of this password "enhancement" system, I think I personally have much weaker passwords."

Assuming that "XX days" === 'less than 100 days', I totally agree.

We use a six month password cycle at work, and I think that's reasonable as it only takes me a few days to remember a password that I use tens of times a day. If it's a password that I use less frequently or a change is mandated more frequently, then I would do the same as Bruce and use something more obvious or only make small changes to the password each time.