I belived that is would come naturaly and that is why my inability to write perfect prose on the first shot would distress me so much. Only the actual act of writing can bring out your inner voice. By choosing words and deciding where to use them do you find the way you like to say things in your writing. To get to your inner voice, you have to peel back the layers of junk and copies of other writers until you get to the real thing. You can use your journals in several different ways to acheive your goal.
Many people use it as a diary.
They use the pages to get out all of thier emotions that were pent up during the day. That is what I use my journal for. You can also use your journal as a mini writing class. By devising assingments for you to do you can get your creativity flowing for your other writing projects. I use a seprate book just for these such assignments.