I was watching a guy repair a wooden boat on a beach in Thailand one day. He had some glue and a thin rope he was stuffing into the spaces between the slats. When the 25 year old wooden doors in our house started to dry out and crack. I tried caulking and such, but it did not last long and popped out.
I decided to try the boat repairman's technique and it works very well. In 6 years only one crack has had to be redone.

Updated video:    • Wooden Door  Fixing Cracked & Broken...  


MATERIALS:
Natural fiber string (cotton or similar)
wood glue
caulking

20210524: UPDATE It's been 8+ years and the joint is still intact, has not split and has not needed any more work.