Ok so I hope this isn't a repost, but I havent seen it on this site so here's the tip-
On every single one of your alts you can make your own guild (for example 3 alts w/3 different guilds). Which opens up multiple guild banks, depending on how many alts w/unique guilds you have. It doesn't cost anything to create a guild, and it's the best way to store lots of items (gems, alchemy resources, etc). They never expire and it doesn't matter if you are inactive on that alt.
Cheers!
,chron