I must confess, I've never been one to keep a diary or journal. I think I tried it when I was 12 or so. I became frustrated with feeling like I had to write in it every day, and would find myself writing about the weather and other things that felt trivial.
There are a few things that I feel like I have done in my adult life that have so impacted my spiritual life, I can never go back. One is keeping a spiritual journal. I keep it by my bed, so that even if I wake up in the night with a prayer or dream, it is handy to write in before the image passes.
I only journal about things that seem like they might be significant...for example, if I feel like God talked to me, or I am struck by a scripture but don't know why, or if there was a breakthrough in prayer. Sometimes I journal about dreams that seem important even if I don't know why. Every now and then as I'm praying, I read through the journal.
Why is this important? Because God is a God of the ages. Process is important to Him. He is a generational God who speaks to us about things when they are still "not yet." For example, Abraham had the word from God that he would have a son and his descendents would be as numerous as the stars. Abraham would have to wait 25 years to see this word fulfilled. Joseph had a dream that his brothers would bow down to him. He spent the next 13 years as a slave and prisoner before being promoted to the palace. David was anointed king about 10 years before he became king.
Sometimes when God speaks to us, it seems like the exact opposite happens. We need the word of God to hold onto like a treasure, to pray over it and meditate on it. I find if I don't write these things down, over the years, the words of God to me fall to the floor. I forget them or can't hold onto them when the exact opposite things are happening. If I am meditating over my journal, I am always keeping the word of God in my mind and heart. I start to see themes of what God has said or is doing. It helps me have faith for the journey, the process, the walking through day to day things.
I encourage you to start a journal. Sometimes you might write in it every day, at other times, not for months. But when you are not writing...read the journal and remember. Keep God's words alive in your spirit.