You can't wax it. Keep it moist with some snow. Also
Helps. That's what even some of the bigger parks and contests use. Well the contests don't run boxes anymore so they don't, but used to sometimes if needed. Doesn't last forever, but if it's just you shredding the backyard it could help.
Also steel coping so you aren't just digging into the top sheet if you're gripping the edges. the downside is getting it flush with the height of your topsheet.If you're rocking 2x4 or 4x4 frame it's not that hard though. You can add 2x4s to the side and add either angle iron or steel tubing. Just drop down the 2x4 on each side the height of whatever you're using so that it comes out flush. Also if it's tubing you can sink it into the bottom 2x4 as well as the side to make it rock solid. Could even bolt each side together through the rail to make it stupidly solid. Angle iron shouldn't really rock at all on you.
I worked at a place for a couple years that had sticky top sheets. Had to keep snow on them all the time as it was sunny as fuck. If they were dry with no water, you could just stick sometimes and eat shit. As long as we kept a tiny big of snow on there to melt and keep it wet, they slide buttery.