I go out of my way to wear down the rough stuff on my waccom so it's as slippery as possible actually. I think because I learned how to draw on a tablet with a really cheapo thing and now I prefer it.
As for slip, my pen has a rubber-like grip to the handle. I know you can buy those for pencils, maybe it'd fit on your xpen?
Also, they do sell removeable screen covers that are rough, so you can put them on your tablet screens. In case you just hate the slippery, (which I understand, everyone's touch is different for a tablet)