In general, the bases you raid should have around 100k Gold and Elixir and require around 30 Goblins and 30 Archers. Your Barracks should be constantly pumping out Troops, two Barracks for Goblins and two for Archers. To improve the amount of Troops being produced at a time, you may want to also have Minions training at the same time if you can afford it.
When you are looking at resources available in a village, keep in mind how much your attacking army costs. Two good figures to keep in mind are the total cost of your whole army, and 1/3 of that number. This can be particularly helpful when farming for elixir, so you dont make negative progress from a botched assault. There is a pretty good calculator for pricing out different army scenarios here. In particular, be very careful about use of spells when farming for gold. The loots need to be pretty phat for spells to be worth it.
Since Im sure you are following our advice and overtraining, its a great idea to keep in mind how many troops youll return to. If you are in an active raiding session and you are leaving your village without full camps, you can count on troops training that full time. That could mean ~5 seconds looking for a target + 30 seconds planning out your attack + 3 minutes beating on your unsuspecting victim. Your mileage will vary depending on various factors, so I consider 3 minutes of troop training to be a good typical value. So for example if youve got four Barracks, that means you can spend 28 Archers on a raid and return to your village full and ready to raid again immediately. I look at these troops as disposable farmers for quick smash and grabs on exposed resource collectors.
Seeing an enemy base with a large cache of resources can present a psychological challenge during the tense 30 seconds you have to decide about whether to attack. When farming for higher level upgrades, it helps to focus on one of the three resource types as a goal and make that your singular focus for several reasons:
One good raid of any resource is unlikely to buy you anything but Walls
Your builders get tied up in long build times, and you may not be able to spend more than one resource
Having a little of every resource makes you a target for all other farmers, not just those seeking one resource
Spending too many troops on a resource you cant use may cost you the chance to harvest goal resources on your next attack
For all these reasons it pays to be single-minded and pass on villages that dont have much of your goal resource, even though it hurts a little.
The difference of a few hundred trophies can have various impacts on your farming effectiveness. For example: Certain resources are much more prevalent in certain ranges The difficult of defenses in opponents villages tend to vary by trophy range Once your farming session is over and your village exposed, the goals of other attackers will vary based on trophy range
Once you find a range that seems to be working for you stay there. Since you may lose trophies frequently when focusing on resources, be sure to mix in the occasional raid for trophies to keep you in the range. It is easier to do a little bit at a time, then all at once when you realize you are hundreds of trophies away from where you want to be.
In many ways Clash of Clans farming raids are like a poker hand played against another player. Neither of you know exactly what the other is packing when the raid starts, money is on the line, and there is an element of chance even when one side starts with the advantage. As time passes during the encounter, what once seemed an easy mark might become a bigger challenge then you expected. In these cases, a good farmer knows when to hold em (with spells or larger or more expensive troop deployments) and when to fold em (minimize losses and move on to the next raid).