What you suggest should be required of every culinary student!
The closest I come to keeping a diary is my kitchen calendar, where I list each night's dinner menu. It mainly serves to jog my memory -- what did I do with the leftover roast lamb? for example. Otherwise I just try to remember how I did something before, and build on that. Not the most scientific way to go about it, but I'm lucky to still have a good memory (for the failures as well as the successes).
I DO go into far more detail when I list my party menus, such as what I used to marinate the olives, or what went into the smoked salmon mousse, etc. Those are also the times that I note which wines were served. These either go into the "Meals Eaten" file on their own, or into the calendar, which ends up there eventually.