Get your feasting hall, and your rally points up. This will allow you to send out several troop movements at once, and since attacking troops must have a hero with them, it will allow you to house enough heros, to send out several attacks at once. Look for high level valleys that are already owned by someone. Most of the time, when someone owns a valley, they don't leave any troops there to defend it. So even if you may not be ready to take on computer-controlled level 7-9 valleys, once someone else has, chances are, you'll find it empty. Take 1-5 scouts, and send them out with a hero. This means it'll only take a few minutes before you take it. And on the chance that someone DID leave some troops there, you haven't really lost much. The farther away from your city you go, the better. A FEW people may get mad at you taking their valleys. I always tell them quit whining and take it back. But some extra distance between your valley and your city means that 1) it might be harder to find your city and 2) you'll have a lot more notice if they DO try to attack you, because anything except scouts takes longer, the farther they have to go.

There's a downside to this. I only attack valleys controlled by the computer since I don't have enough army to do otherwise. I thought I didn't attack any owned valleys, but apparently I did and somebody plundered me.