There have been a number of threads recently requesting ANet to add various forms of new storage. It's very unclear if that's likely to happen, never mind when. So here's a list of eight different options for increasing storage right now, comparing the cost per slot.
Second best depends on what works for you:
Other alternatives are less convenient or outrageously priced
And of course the best option is always: change your inventory management habits: storing less means more time for you to play the game.
I'll go into gory detail about these options in the first follow-up post.
"Face the facts. Then act on them. It's ...the only doctrine I have to offer you, & it's harder than you'd think, because I swear humans seem hardwired to do anything but. Face the facts. Don't pray, don't wish, ...FACE THE FACTS. THEN act." — Quellcrist Falconer