My grimiore, or as I like to call it, my journeybook, was a long time evolving. When I was younger and read too much bad fiction, I was pretty sure it was supposed to be a huge dusty tome, full of archaic script in faded ink - recipes with ingredients like frog's tongue and eye of newt. I was not planning on keeping one.
But I did need a place to write down the things I learned on my travels - folkloric cures for this or that; uses for herbs, oils or incenses; spells discovered in other books or composed on my own. I tried a lot of things - the computer, 3 ring binders - but have finally developed a system that works for me.
I write everything down in a journal. The current incarnation has polo ponies on it. No particular reason, I just had it around the house. Waste not, want not. Every time I start a new one, I copy information from the old one: I keep a section with lists of oils, incenses and herbs and their uses. Tried and true spells. Words of wisdom. Rituals. Prayers. Rewriting these out every year or two may seem a ridiculous waste of time, but I believe that it helps me to learn this information by heart. In time, I want these things to be a part of me, an intelligence I can rely on at all times, not just when I have my Tome of Magikal Lernings and Informations Moste Importante at hand. The rest of the book is for my own journaling. When I devise a new spell, I record it, and then I also try to record my experiences or observations of things relating to the spell working. I am trying to become more sensitive to the world around me and my own influences upon it. I do the same thing with Feng Shui, which I've recently become obsessed with. Every time I add more black to my career area, for instance, I record it and then record any observations - like throwing a pebble into a pond and seeing how far the waves reach.
Rituals, ideas for honoring my pantheon, recipes for teas or tisanes, etc. Words of wisdom from my grandmother. A sighting of a wild animal or a strange experience that feels otherworldly. All of these things go into my book, so that I can remember them and learn from them. Once I fill up the book, I keep it and come back to it every once in a while, returning to an experience and re-examining it anew.