I have found clear nail polish good for strengthening ball joints. I've used it only in the past few months so I can't say if there are any negative long-term effects.
I also don't transform my figures a lot but I would guess that repeated transformations over a length of time may wear nail polish layers down.
The nail polish I use stinks a bit so any figures I use it on are left out to air before going in the display cabinet.

I prefer using nail polish than super glue because the effects are worse if you stuff up super glue (I have glued my fingers to more than one figure )