Attacking valleys is a great way to generate resources, but to get resources you have to attack the valley without conquering it. If you attack a valley with your valley slot full then you wont be able to capture the valley but you will get 300-600k resource from that valley.
Again if you capture a valley and then release it then the valley will regenerate the resource and troops within it. Thus you need two cities near a lvl 10 forest or hill.
1) Send 30k archers, 1 work, 1 war, 1 sc, 1 pike, 1 sword, 1 cav, 1 phract from the first city with valley slot full. As the valley slot is full this attack will not be able to capture the valley but will get the resources.
You will get 300 K to 600 K lumber or iron from each attack.
2)Send another attack with 1 scout from the other city (with a open city slot) to capture and abandon the valley. This attack will regenerate the Valley immediately with the source and troops.
The big wave will bounce back, but will loot the valley, then when your scout lands it will capture the valley. Immediately abandon the valley.
Repeat the waves in sequence with camp time of 1 min, then with every attack from the first city you will get you 300 K to 600 K resources reach time.