I use a past - present - future journaling concept.
The way it works is every day I write down the date.
Under the date I can put in information about what's happened since the last journal date (i.e. I met a client and discussed ____) that would be my past.
Present is when I'm on a bus or standing in line, etc I can record what's going on currently in my mind or what's recently just occurred and ideas.
For future, I write down ideas and brainstorms on what I am wanting to do, how I can overcome hurdles etc.
Note: I don't force myself to record all 3 categories. It's just a nice target to aim for. If I only have ideas, that's all I record.
I recommend you be flexible and have fun with your journal. Stiff journal rules will make it a chore.
Enjoy!