This question has been answered here.
The basic conclusion is that, apart from keeping the drives in a recommended temperature range, I'm not sure that there is any great way to prolong the use of the hard drives. There has been reseach into how often hard drives last when not spun-up regularly vs when spun-up regularly but it appears mostly inconclusive.
Regardless, it might be better to ask what is the best way to prolong the use of the content on the hard drives. And the best answer in this context (the context of bit preservation) is to make more copies. If you have done that then if a hard drive holding one copy fails then you will still have access to the other copies of the content.