What shoes do you have? When I first started stand-up tricks, I had a pair of shoes that were nearer hiking boots than proper athletic shoes, and I do seem to recall having problems with this once in a while. Another pair of shoes I got from the thrift store didn’t work at all for unicycling, because my feet slipped off the frame whenever I tried gliding, let alone stand-up tricks. I now have a pair of Converse High-tops which works great for stand-up ww/glide/koosh-koosh.
Maybe you should consider getting a new frame. The one on the Koxx-One freestyle unicycle looks great for stand-up tricks. The Nimbus II frame (which I have) has a similar fork, and it offers great support.
Leo White put little rivets into the crown of his freestyle uni for grip. Alternatively if you were feeling super-clever, you could get someone to drill + tap some holes for a thread to fit allen key pedal pins, which would have the same effect, except you could replace them if you wore them down.
Grip tape is okay, gives very good grip, but it’s a bit mingy. You’ve got to be really careful not to have edges free anywhere or else it comes up and looks really rubbish. The rivet thing looks much much nicer.
If you have access to a welder you could eaither weld on small pieces of trip as on the Kh '04 frame or ‘noodles’ (i think they’re brake/gear cable guides) like on the KH '05 frame. Both of these were originally specifically designed to aid grip of the foot on the crown.