I have kept a journal that I made myself (I'm a self-certified book artist). I included some random paper that I accumulated over the years that are meant for drawing, but I won't use for that. I collage images of my veggies into my yard, to give myself a visual, or I sketch what I want it to look like.
After a few years of keeping it, I am finally seeing the benefits. When I first moved into my current home, I got really excited about starting my tomatoes in my southfacing windows. I think I started them in March ( I wrote it down) and had 3 foot plants over taking my house. (I bought nice seeds from a Cooks Garden catalog, and they all took, so I had near 60 plants). Now when it's April1, I can tell friends and family to chill out since I haven't started my seeds yet. I have 2 months to develop a healthy plant to put them in the ground on June 1. And i also make note that my tomato production is just as crazy as the neighbor who prematurely put their plants out on May 15th. I make notes from the extention service that may be helpful in upcoming years (like my area may have changed zones last year!) I also sketch out perenials and possible rotations for my veggies. Soon, I'm going to bind all my empty seed packets... and start saving seeds in them.
Anyone can keep one of these journals, whether you purchase one or make your own. Just don't think it has to look like a Martha Stewart or Davinci journal/sketchbook. Have low expectations, and it will come together. A friend of mine keeps a garden journal, and 1 journal took 10 years to fill. And she's on her 3rd one now. Want to see my journals? go to www.mnartists.org/Heidi_Steadman I will scan in images sometime soon.