I keep a whistle on my jacket zipper pull. I don't like the odds of me being able to dig in a pocket and get the whistle to my face without losing it.
I don't even know how much it helps to ski with a partner, unless you're staying exactly in each other's tracks at exactly the same pace - and then, will you notice when the person behind you disappears?
I don't know if it really helps, but I've been doing decline situps at increasingly upright angles. Maybe it will help me get out.
I got partially stuck in a tree well last spring. These were in very thinly spaced trees - not even a glade, really - and even though I wasn't fully submersed, my feet were in the snow uphill of my upper body, and it was hell on wheels to try to pop the binding or get the ski to a more favorable position. So I started yelling for help, but even though people were skiing a well populated traverse maybe 10 ft above my position, they couldn't hear me until I blew my whistle, and even then were confused for a bit. Without them, I felt fully stuck - in good repair (except for a knee strain I noticed later in the day) and with my head and upper body fully out of the snow, but still stuck. It was disconcerting.
I suspect that if you're skiing trees, you're accepting some level of risk, even if you're very careful.