A good 'camera/opticians' lens cleaning kit is best, (along with a good quality 'camera lens type' blower brush), as Lee has suggested.
It also sounds a little like you have been putting them away with a bit of moisture present. I would suggest going to your local 'K-Mart/shoe store', & asking them to save/give you, some small bags of 'silica gel'. Every boxed pair of shoes has some. I ended up with a 'shoe box full'.
One or two can be placed inside your E/Pc boxes/bolt cases or whatever, to remove the moisture.
Do not put away wet/damp...
HTH...
L.
ps. Take them out every so often & let the sunlight & air at them to help kill the fungii that may grow inside them...
