How do i get cheaper insurance when i have driving convictions?

As others have said, you normally have to disclose convictions up to 5 years old, no matter how long they actually stay on your license. But you may have to declare them for much longer than this, as most insurers also ask if you have EVER been banned from driving - so even if it was 20 years ago you still have to tell them. They then assess whether you are likely to re-offend and load your premium accordingly.