Temperature.
Temperatures of 30+C but not above 45, seem to suit most seeds.
A propagator is useful, but an airing cupboard with a hot water tank is adequate or even better, as ordinary propagators have difficulty in maintaining the high temperatures in the winter. Recently I have built my own gash propagator from an old wall cupboard covered in polystyrene sheet for insulation, a heater cable and a thermostat.
Palm seeds like high temperatures in general, but there are some exceptions I've come across. Trachycarpus fortunei and all the other Trachycarpus species seeds I’ve tried germinate best in temps of around 25C. Or a little less. Room temperature almost.