Instructions
Sell off things you don't want or don't need. This works well if you have a load of Technical Machines that are not compatible with the Pokemon in your team. It is also a good way to make room for a major purchase, such as that of Ditto from the Celadon Game Corner.
Store things virtually using Ash's personal computer storage system, which you can access at any Pokemon Center or at Ash's mother's house. Items you place in here will remain until you come again to retrieve them.
Drop useless items in tall grass or along a path. This is an especially good option for items like the Bike Voucher that have no utility and that shop owners will not purchase from you.