This is the email I received from Greg Godfrey after I asked Tethrd that same question. It really helped me out and found that it stopped riding up any more. Hopefully it will help you. ;
From Greg,
Try these things.
1. Move your tether down around neck/chin height when standing straight up. Most people find the sweet spot as low as their chest and as high as slightly above their heads.
2. When you're in the "sitting" position, your knees/quads should form a right angle to the tree trunk, or be slightly above a right angle...but not much. If you have too much angle in your knees, the "slide" feeling can be exaggerated. To adjust your angle move your prusik knot (or Ropeman 1) lower on your tether until you hit the proper angle.
3. Make sure the top webbing strap (the one with the molle loops) of the Mantis is just below your belt line. This is wear most users find the best comfort. Also, don't be afraid to move the main webbing straps of the Mantis higher and lower. If #1 & #2 don't fix your problem, play with the positioning of the Mantis until you get comfortable.
4. Add in the Mantis Recliner if you haven't already. For me, the Mantis was comfortable, but not FANTASTIC until I added the Recliner. Now I can sit all day long and be completely comfortable. I'm not sure why back support makes such a huge difference in the comfort of my butt, but it certainly did. The Recliner is a Game Changer. I'm happy to provide a Recliner at a discount if it will help you get comfortable.
5. Don't forget Saddle Shape is a thing. It can take many hours before you feel totally comfortable. It took me probably 5 sits in my Mantis before I got it figured out and I have almost a decade of saddle hunting experience.
Ernie, feel free to chime in if I missed anything.
Hopefully that helps!