The Microsoft Store is currently holding a trade-in promotion that gives up to $400 credit towards the purchase of an Xbox One X.
Eligible systems for trade-in are the PlayStation 3, PlayStation 4, PlayStation 4 Pro, Xbox 360, Xbox One, Xbox One S, Nintendo Wii and Nintendo Switch.
The promotion offers a flat $100 minimum credit value, regardless of which device is traded, so even if you still have an old Xbox 360, Wii or PS3, it may be worth putting towards an Xbox One X. However, only one console can be traded towards an Xbox One X per customer.
The Xbox One X trade-in promotion runs until August 29th, 2018.
A list of Microsoft Store locations can be found here.