I think it's a good idea to keep a photo of yourself as a child where you see it often, especially one that reminds you of a happy time. I have one of myself about 3 or 4 years old, lying in a hammock with my grandmother and both of us wearing big grins. I have a couple of cute snapshots of me as a toddler on Christmas morning that I like to put out for Christmas, along with a plate my son made that says, "Merry Christmas mom and bad"
The best thing for bringing out your inner child is spending time with a child. It's very instructive (and entertaining) to watch how they operate kind of like people but without the filter of inhibition.