They make zipper wax for this purpose. I use a bar of soap and it seems to work, but I've never tried the wax, it might work better.Since you mentioned it, have you actually ever used WD-40 on the bag before? In particular, I'm thinking about using WD-40 on a rogue 25 zipper pull. Of the two zipper pulls, one is very stiff and hard to move along the zipper track, and I’m hoping that something like WD-40 will help it.