To edit those forms, go to the normal "edit layout" for the object in question (for instance, Contact in this example). At the top, click "mini page layout":
On the next window, select those fields that you would like displayed when doing a "mouse over" a contact in other objects.
Once saved, you'll see the fields listed on the right when highlighting the contact name with your mouse. This of course works with recently viewed, activity lists, calendar, etc.
What I like to do is highlight (when displayed), then do a "control-C" to copy that to my clipboard.