I am a terrible Forgetful manager. It is the one skill that I still have not made much improvement on in the 14 or so months that I've been a manager.
A Forgetful manager understands that nothing will stick in his head. He realizes that trusting his short-term memory is only slightly more foolish than trusting a junior developer's estimates. The Forgetful manager documents everything that needs to be documented. Project meetings, manager meetings, developer meetings, requested salaries, actual salaries, past interviews and your wife's favourite perfume are all good things for a Forgetful manager to document. In particular, if you learn something new and think 'I've got to remember that', then you might want to find the nearest pen and scrap of paper to document that thought before it escapes into the 95% of your brain that you don't use or have killed off during those team-building sessions.
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Of course, we all know that managers use at least 2 or 3 percent of their brains more than that. But it is definitely true that all managers forget things if they're not documented. It's inevitable. After all, there is so much crap getting chucked at you during every waking moment that your brain comes up with a great system for dealing with this information overload: it dumps data.
Dumping data is a good thing. Every good system should perform a data dump on unnecessary data. However, your brain isn't too good at separating useful data from the latest load of crap that you've picked up from DailyWTF. That's why you need to take the time to write stuff down.
Grab a tool like WikidPad to act as your impromptu memory map. Or implement a full-fledged process like Getting Things Done. Get yourself a Moleskine notebook and a Fisher space pen. Maybe even create your own low-tech PDA. Or just start organizing the crap that you've already documented.
Whatever it takes, starting to acknowledge that you're Forgetful is the first step. Realizing that it is inevitable and dealing with the consequences is what will help you to manage effectively.