There's several plugins out there for the storage of EXP.

My personal favorite allows you store small amounts of EXP in bottles (bottle o' enchanting) and 30 levels of EXP into buckets (buckets o' enchanting).

This allows for EXP to be traded and such, and people already try to sell 30 levels of exp so it'd be nice if they could do it in item form.

There's also mods that allow you to store as much exp as you want into a bookshelf, it's only for personal use not trading but it's also nice.