Unused miles are marketed as dollars in a bank. But they're not. They're unsecured claims against the airline.

Worse, they're claims the airline reserves the right to unilaterally devalue. They're deposits in a Venezuelan bank.

As such, the best strategy is eagerly consuming miles. This is admittedly not always easy to do, particularly mileage flights can't be paired with upgrade certificates and do not count towards status. In my experience, frequent fliers with lots of miles and who care about maintaining status are best served by buying others flights with miles.