Watercolor pencil has to be one of my all time favorite materials to use in art in general, but I love using it in the journal. There are several basic techniques to use it, but one fun way is to use it with watercolor paint.
By coloring the pencil down first, and then painting a contrasting color over top of the pencil, you can get some gorgeous color blends and gradients.
This technique works on blank pages as well as a layer on top of other materials.