When?
First consider the Gold mine size. Plains of Snow has a lot smaller mines, 40k as compared to a more "normal" map such as Garden of War which starts with 60k mines. You will find that 40k mines run out about midway into the Stronghold/Keep stage or just into the Fortress stage which will leaving you looking for more Gold much earlier in game play. On maps such as Plains of Snow getting an expansion mine is often make it or die. On maps with smaller Gold mines you will have to expand earlier.

Going for a second Gold mine too early can be very dangerous if done too early. A Town Hall is 1200 800 which is a lot of resources to spend. Resources spent on a Town Hall gives you no added offense which puts you at quite a disadvantage in the offensive race. The other player may spend their resources on Grunts and using them to attack in which case the resources spent on second Town Hall will do nothing for you. However if you are able to set up that second Town Hall and get Peons mining Gold and harvesting Lumber you will be able to train two Peons at once (one from each Town Hall) which will allow you to mine Gold and Harvest Lumber at a much quicker rate than someone with only one Town Hall and Gold Mine. The problem however is getting that second Town Hall set up, going, and defended while still defending the main town against the enemy who may be using one mine and devoting all resources to attack units (Grunts). Things that can help here are wall-ins and Towers. Another strategy that sometimes works is to build up a group of Grunts or Ogres then use them to escort a group of Peons to protect them while they head from the main town to the new town, build, and begin mining. This can sometimes leave the main town open to attack but wall-ins and Towers can protect the main town until the new town is up and running. Another strategy commonly used is to just send a few Peons to build a new Town, using 1 to build the Town Hall while the others build Towers or begin to wall the Town Hall in using Farms and buildings. Yet another strategy is to just send 1 or 2 peons over then hope the enemy doesn't find them. This is very risky and should not be used unless you are forced to.

Where?
Sometimes setting up a new town, goes deep into the realms of psychology. There are so many options.

Build in a dropped/eliminated players town. Sometimes people forget about the mine and town after a player drops or is eliminated. They often check out all the mines except the mine of a player who has dropped. So when a player drops or is eliminated, go set up in their town. Often your buildings blend in with the other buildings that were already there, especially if they are the same race of buildings. So the enemy may fly over and think all the buildings belong to the player that left. This quite often works.

Once your town or expansion town is destroyed, go back and rebuild it in the same place. Usually people expect you to go find another mine, because that is what most people do. They won't expect you to go back to the same place because they know you know they know about it. But since you know that they know that you know that they know...... they possibly won't go back to check in a town they have already attacked and cleared.

Sometimes the last place the enemy checks is the resource spots right next to them. Often they assume you would never be as so bold to build in "their" territory. Sometimes the safest place to build is right next to the enemy but you run the risk of the enemy finding it when trying to expand themselves.

There are trade offs for building close or away from the main town. If you build at the closest resource spot to your main town you can more quickly send reinforcements from your main base to protect it because you most likely have more Barracks at your main town. On the other hand people often expect you to build at the closest expansion so they will most likely be checking and ready to attack it. Building further away sometimes gives you a better chance of hiding until your new expansion is defended and under full production but you run the risk of the enemy finding it early on by mistake or design.

Hide the Peasants or Peons
If you've reached the Gold Mine at which you want to build your next expansion town but are not quite ready to built yet (normally due to lack of Lumber), hide the Peons behind the mine. When players check mines, they often don't check all the way around them and will often not see units behind mines.

[ Click to Enlarge - 102 KB ] Peons hide behind a Mine.

Town Hall Fake
Most people build just one expansion town at a time, but instead build two or more. If the enemy finds one put up a big fight for it acting like it is the only thing between surviving or dying. Meanwhile use your other expansion towns to build up and mine Gold and harvest Lumber. If the enemy does get this bait Town Hall you will still have one or more fully underway. The enemy may focus on your main town thinking that is all that is left which should be defended in the early part of the game.

Don't Give Your New Town or Towns Away
Delay your enemy finding about your new town as long as possible. Do not give your opponent any reason to think you have another town. Try not to lead enemy units from your main town to your expansion town if possible. If you see the enemy has Flying Machine or ground units following turn another direction to lead the enemy away from your expansion town. Also when a Town Hall is destroyed the Peons harvesting Gold and Lumber will then head to the next nearest Town Hall or Lumber Mill (for Lumber). The enemy will most likely know where your main Town is and if they see these Peons heading to another direction (your other expansion or new expansion) they will know what direction to head to find your hidden bases. When your Town hall is about to be destroyed, select the Peons and stop them or head them in some direction other than where your new hidden bases are located.

Game Over Feint
You can type as your enemy is destroying your old town something like "Ok, you won, I'll just let you destroy my town". This trick often works but not against experienced players unless you catch them on a bad game. Often once someone thinks they have won they stop building units and begin messing around. If you are building up a force of units in secret while they have pretty much stopped production and are using what they have already to hunt you down you can build up a large enough force to launch a surprise attack on the enemy. In some games such as force has turned the tide of the game and allowed the defeated player to win.

What You Can Learn
Always assume that someone has a new town when you are destroying their first town. If the town you are destroying is not their only one, and you stop building, upgrading, and expanding your town you could lose. Don't stop mining, building, and training units continue on as if they were still in the game. Never assume a player is dead until they actually drop or surrender. Don't listen or believe anything what your enemy says about being dead until they actually quit. Good players will do anything to make you think they are dead so they can recover. Build plenty of Flying Machines and check all in and around all of the Gold mines to make sure the enemy is not rebuilding.