Just put your stuff in a chest or something alike. If you drop it to the floor it will vanish after some time, but it is save if you put it in some container.
There are some exceptions: If you are in an area which changes after a completed quest then all stuff from chest, tables etc is gone. Example freeing de Arnise keep from the trolls: if you put items in a chest before you kill all trolls and free the castle, then these items are gone after you complete that quest. But after that it is save to store equipment there.