There are various mods and tools for this. Personally I have been using GD Stash for years, since it is totally reliable, grants unlimited storage and a nifty search function. Some dislike GDS, because it can be used as a cheat tool, but no one is forced to use those functions.
GDS is a third party tool. It stores your items outside the game.
Stash mods usually increase the size of the ingame stash by a lot, which I don´t like very much.
