So far, double socks - thin dress or polypro socks on the inside, thick squishy fleece ski socks on the outside - is working really well for me, even on low level jumps. Super-comfortable, warm, just feels wonderful.
That really snugs up the boots, and I made custom insoles and did a heat mold first, so they fit my feet perfectly. Maybe you only get slippage when you have a poor fit, leaving room to slip?
And it lets me, who can't do more than half rotation jumps at all well, mange to use a pair of slightly used, oversized high level freestyle boots. I bet they never wear out.
Everything - strokes, edges, cross-overs, progressives, turns, spins, jumps - works a bit better than on my old broken down Ice Dance boots that I used with thin socks. The only things I don't love are the 1/2"-short-for-my-feet Wilson Excel blades I've been using with them - I keep scraping at the tail, slowing me down, and twizzles are harder to balance. Now that I'm happy with the new-to-me boots, it's time to move the better-length-for-me Matrix 1 Dance blades from the old boots to the new ones.
This experience is completely changing what I thought I knew about boot fit. If I am willing to spend enough time altering the fit to work, and wear thick double socks, I can be very happy with too stiff, too large boots. (But I'm still hesitant to tell others to do the same, unless I am sure they will take the time to make them fit.)
Wippee!