The problem with the flare is that what works for some people does not work for others. Unfortunately it's a judgement thing and as a result difficult to teach.
The best trigger I use for the flare is once they cross the threshold is to transfer their vision to the end of the runway. This helps keep the aircraft straight through the whole flare/landing business. Then to obviously keep the aircraft descending. When the runway edge, which they are noting in their periphery, is up to their shoulder level, close the throttle and smoothly pull back on the control column. Then they should keep the runway edge in the same perspective using progressive back pressure until the wheels settle down nicely (on the centre line if poss
). If they pull back too hard and it's not a balloon, then to hold that back pressure until they have the runway perspective again at shoulder level, and then apply back pressure as above. The control column is treated like being on a ratchet and should
never be pushed forwards in the flare.
If it all turns to poo, full power, carb heat as required and climb away.
All of this is first demo'd with the patter and the stude following through. If after a few attempts they still haven't got the hang of it another demo.
Like I said, it really does depend on a person to person basis. Some people take to it like a ducks to water others need a bit more work. However, in the end everyone get's it right so keep plugging away.