MM8 automatically places markers with notes on maps at locations where you've found teachers of the various skills. Please note that for some reason the Expert Bow trainer does not get marked. You can, and should, add your own markers to help you revisit others special characters such as Quest-givers and Traders.
You'll also want to mark the location of characters who may eventually join your party. Sometimes you won't won't know who they are from their conversation, but you can usually identify them by the background behind their portraits. In all but two cases it will match the background around your current players' portraits.