Training for the corpus callosum?
It's been known that Increased Corpus Callosum size was found in musicians who began music training before age 7 and that Training working memory leads to growth in Corpus Callosum.  As for how to train working memory, that's a diverse subject.  There have been a lot of experiments involving Training working memory for ADHD patients, however a meta-analysis has revealed that most of the training from these ADHD-centered programs only leads to short-term gains, not long-term or generalisable gains.

To speculate some, I think taking on new tasks and challenging yourself repeatedly in a wide context (as opposed to isolated tasks "designed" for working memory) is your best bet.  Learn a new instrument, learn to program in a new language, learn new regions (spatial mapping is associated at least with hippocampal cell growth as long as you use spatial strategies, not procedural ones).