Go into your inventory and look at everything. Ask yourself if you've used the item to craft something in the last week. If not, get rid of it unless it's a rare and expensive item.

Don't store what you can buy from a vendor. People forget that shards/crystals are still need to make the intermediate step crafting reagents and they have a value. Buying from a vendor frequently ends up cheaper.

Don't store what is consistently plentiful and cheap on the MB.

Don't store what you can easily gather yourself in the quantity you immediately need.

Don't forget the Curious Crop exchange. Even if you're not growing the Voidrake/Althyk Lavender yourself, buying it from the MB can be cheaper than buying the ore/nuggets/etc. and a big time savings that you can be using to gather other things you need that can't be bought from a vendor.