Clenching is a habit to avoid wetting your cloth etc...
Wet your pants (/diapers) in several (safe/unsafe) situations and give a heck on consequences for your cloth, your shoes, your floor, bed, chairs, the street, playgrounds, etc...
Then you'll get used to wet your diapers (/trousers) more easily fast.
Wear black trousers - go for a walk - wet in your trousers (or in your diaper until it leaks).
Nobody would notice, but you'll get used to wet trousers and thus get used to wet your diapers more easily, being not as afraid of wetting yourself as before.
Therefore you'll wet your diapers more easily in future.
If you like to reinforce:
Do it public.
Reward yourself immediately after wetting! (with sweets, listening to your favourite song, smiling, etc...)
Tell yourself your own urine is not a bad thing to come and stay in contact with. And it really is not!
Alcohol is very helpful.