In reply to jim jones:
When mine start to look 'experienced', they must acquire a certain something that is 'just so'. At this point, the dog takes them to a quiet corner to improve them. The last pair lasted two months before improvement was necessary. He has so far improved two pairs of approach shoes, one pair of running shoes, one pair of squash shoes, two pairs of carbon-soled cycling shoes and three pairs of high heels (not mine). He is happy to bide his time until opportunity and need for improvement coincide. If the shoes do not meet his exacting standards, he leaves them a while longer until they are, as above, 'just so'.
The obvious solution would be to teach him a lesson he won't forget. However, castration and shoe improvement seem not to be correlated....