Clear policy
Helps a lot, I am in the middle of the staff pc updates. If they know that there is a four year replacement policy then that helps. Mind you I supported one manager who ensured that every time someone in his department got a new unit he had to have it and they got his new old one, even with a strict policy. This lead to a pregnant staff member getting an older desktop replacement while he got her ultralight, he looked so selfish and not to mention petty while she lugged that brick around.