As for me, I consistently get +2 TU, +1 STR (weight bar with red to be sure), +1 ACC (at least one shot), +1 RFL (at least one reaction fire), +1 BRV (at least one panic/hallucination/fleeing state, either induced through psionic attack, or natural demoralization, but alien control doesn't count).
At first, I got some +1 TU on small maps for static soldiers (e.g. machine gun), then I used to make them running before/while the final assault.
I believe I got once +2 BRV (not through promotion), but I'm not sure. For instance, 5 morale events are not enough to get +2 BRV, 7 kills (and more shooting) are not enough to get +2 ACC, etc. So yes, what's the threshold?
Perhaps, I have to wait for über-soldiers, so that a handful of them could assault a carrier alone, thus getting more opportunities to shoot/reac fire/panic...
But this is not really a concern, because, after 30 missions, having +30 instead of +60 on already decent (50-60) starting stats if fairly good.