I have all my DoH and DoL at 40+ so you can imagine how full my retainers are.

What you simply HAVE to do is check each item on one of the database sites out there (I use gamerescape) and get rid of anything you will not need any more.

Find out how much of an item you will need in the future and keep just over that amount (for catastrophic failures etc). Don't hoard stacks of raw gems for example.

For culinarist especially, they need so few of each item that it might be simpler just to harvest or buy what you need for each crafting level.

Also, don't store items you can buy cheaply from the crafting guilds: i.e. rock salt, iron ore, cotton bolls can all be bought cheaply from the crafting guilds.

I also tend to only store raw materials. For example I make ingots as I need them and keep them in my inventory if I made a few too many and I know I will use them soon.

I use the first tab of each retainer for items I'm selling (i.e. finished goods or materials I no longer need that have some value). If that tab is full then I stop crafting till I've sold some stuff.