What physical form does your journal take? Spiral-bound, cahier, oilcloth sketchbook, Moleskin, etc.? What materials do you use to fill it? Pen, pencil, charcoal, paint, collage, etc.?
My preference is a hard-bound notebook with a black cover. This is what I was first introduced to in 1980. I like to be able to use both sides of the page. I am not concerned about the manufacturer of the book (e.g. “moleskin”) but I like to be able to use both sides of the page, prefer unlined pages, acid free paper with a paper weight of at least 60 lbs. My favorite sized book is 5-1/2 x 8-1/2 (14x21 cm) though I have used soft cover sketchbooks and one or two spiral bound books of various sizes. The size noted, however, is easy to carry with me almost all the time without drawing a lot of attention to it.
With rare exception, I have written in the journals with a Pilot Precise “V” Series pen. It is close to a mechanical pen in construction, and over time I have found the ink is of a low acid content and doesn’t bleed through the pages.
I have also sketched in the journals pasted in items as keepsakes and made an occasional collage.
Watercolor (using a “dry brush technique) and pen/ink sketches. I don’t use pastel, soft pencil or charcoal because I am concerned about it smearing on the pages.
I’ve made collage pages, and inserted photographs.
Many times (not always) I have affixed things on to the covers of the notebooks. Once I did a collage that encompassed both covers. Often, with the smaller books, the intent is to make distinct one volume from others, or to easily discern front from back.