Another good idea is to get yourself a handy-dandy notebook (see above). It is small, inconspicuous, and personal. You can slip in in your jeans pocket or in your purse. So when you get an idea, or the beginnings of a poem, or ideas for your book, or something you'd like to record, you can take it out and jot down what you need to. At a later and more convenient time, you can transfer it to your actual journal and elaborate on it.
I do a little bit of both. Lately I have been slipping my real journal into my purse whenever I go out, but I also carry a little notebook around at times as well. It all depends.
Do you carry your journal with you?