I too used mindmaps when I was studying engineering. I found that there wasn't much advice on mindmapping mathematical subjects - bit of a problem since I had quite a few.
I ended up mindmapping the processes within the maths using a sort of flow-chart showing the steps I needed to take - reasons and additional info became branches off this series of steps. The steps were literally the steps I would take to derive an equation or to solve a problem. The mindmaps ended up with a central image, as usual, but with little flow charts forming some of the branches showing derivations etc., and any relevant non-equation info put on the mindmap as usual.
I hope that helps a little - if you want further explanation give me a shout.
Cheers,
James
