Q: My 5-year-old Sheltie gets in the kitchen trash, bathroom trash, and bedroom wastebasket whenever I am not watching him or not at home. It usually happens when I am not at home and forget to put him in his crate. How do I train him to not rummage through trash?
A: If you go by the seven dog years per human year age estimate, a dog's habit of five years would be roughly equivalent to a human habit of 35 years. Quitting a habit that ingrained is not easy.
He knows you don't approve of his habit, but sometimes 'forgets' that the trash is off limits. You know he tends to do that, but sometimes you 'forget' to crate him when you leave home. To help your dog quit such a strong a habit, you'll need to be consistent. And part of that is making all your trash containers dog-proof, so it's impossible (not just difficult) for your Sheltie to practice his trash-rummaging habit. You need to either secure the trash or secure the dog -- preferably both. Habits eventually fade when there's zero opportunity to practice them, but they stay strong when occasionally rewarded.
To help your dog give up his undesirable habit, help him form a new one in its place. He needs something interesting to keep his mind and body occupied when he's home alone. His trash habit is like a hobby. It gives him fun and satisfaction to sniff and find all those hidden goodies and tearing them up or eating them. A treat-loaded rubber or plastic puzzle toy would be a safe and satisfying alternative to trash rummaging. Give him two food-puzzles when you leave him alone, and secure the trash so he can't get to it.