Its a very meaningful dictum.
But widely misunderstood.
Its not about yourself doing what you tell others to do. No sir.
Its about the 'before', not the 'after' of preaching.
If you have to preach, give a speech, lecture, etc, you should prepare well in advance and rehearse, practise, etc. Otherwise you will surely make a blunder in the actual.
Its basically about "Practice makes perfect"
The saying, as I was saying is about the 'before' not the 'after'
So "Practise what you (gonna) preach".
Whether after that you "Practise what you preach(ed)" is irrelevant ;-)
(That was just in lighter vein. I dont like hypocrites.)