" Make your cities unattractive for looting. 
If you are being routinely targeted for looting, 
it is because the aggressive player is finding it worthwhile. 
He is getting a lot of precious resources from you. "


To make your city less attractive, concentrate your efforts to build a Storehouse and raise its level as fast as you can. Every level of a storehouse will protect an additional 100,000 of food, wood, stone and ore.
It will not protect gold.
A level 10 storehouse will protect 1,000,000 of each resource from looters.


Use the resources. Make sure you are building and researching in both cities.
An attack by another player will not disturb these queues.
Also, make sure you maximized the number of troops being trained. 
In all of these cases, the game will use your resources to perform the particular task, and will have "protected" them from looting as well.


You cannot hide the resources in the Market - don't try it as it will not work. 
But, you can hide resources on your troops and send your troops to reinforce your wildernesses. 
If you were smart, you did not conquer wildernesses near your cities. 
That would make it easy for the offending player to figure out.


You can also send excess resources to your second city. Hopefully that city is not being looted too. Unfortunately, most people build both of their cities right next to each other.
By doing that, you send an open invitation to aggressive players -
if one city produces good loot, then the second city should as well.
Move one of your cities to another province. 
It will be very hard for the aggressive player to find them both.


Send your resources to someone in your alliance for temporary storage. 
If the aggressive player is not getting any loot from you, he will strike your city from his target list.
After a few days of no activity, your alliance will be able to send your resources back to you with no problems.