I've been keeping a journal almost literally since I knew how to write, although my early childhood journals have been lost to the sands of time. I only have the ones from age 14 on (I'm 32 now). I am currently on volume 41, counting only the ones I still have. It usually takes me about 4-5 months or so to fill a notebook.
These days I write something every day, even if it is only one line. I haven't always done that, though; there are definitely gaps in my journaling.
I've tried keeping it on the computer, but hated it. I always go back to longhand. Computer entries are too easy to lose, or delete, and I don't feel the same emotional connection to what I write when I'm writing on the computer.
For me, the best way to write is just to begin with my predominant mood, feeling, event I'm thinking about, etc., and go from there. Sometimes I write pages and pages a day, sometimes not. It all depends.
I also love to write down my dreams... I write them in pink so they stand out from the other entries. I've been doing this for over a decade -- my journals going back to college age all have dreams written in pink.
I keep an index of dreams by title and theme, so I can see patterns in my dreams.
I also sometimes write poetry or do drawings or scribble short stories in my journal, and I keep an index of that creative stuff too so I can find it later.
My old volumes are filed chronologically on a shelf, except for a few more sensitive volumes I have locked away separately. (I strongly believe in protecting your own privacy if you don't want anyone reading what you've written.)
As others have said, journaling is a very individual thing... this is just some of what works for me.