Do this as often as you can, have time to. This will prevent the drive-train from any damage and the chain from wearing out. Usually the chain requires lubrication and cleaning when it begins to stick, you hear weird noise from it or when you notice that it is dirty. Normal chains look shiny.
Before you lubricate the chain you must clean it from the previous lubricant, which has turned into grease, mixed up with dirt, dust and god knows what else. For the cleaning, cleaning services suggest you shift the chain to the smallest sprocket. For the most of the dust and dirt wipe the chain with a rag, soaked in cleaning solvent. Clean the sprockets as well by using the same rag, or a special sprocket cleaning tool. Make sure you get into the narrow spots as well. Usually they are the most crucial ones.
Now we move on to the lubrication. Cleaners in London advise you to shift the gears into the middle socket for this one. The oil you are going to be using can spread itself, as the chain moves, very well, so do not overuse it. Apply the lubricant along the inner circumference of the chain, moving towards the sprockets on the rear, on the bottom part of the chain. Then run the chain backwards while dripping oil on the both sides of the rollers.
In order to distribute the lubricant everywhere, keep rolling the chain while you shift through all the gears. Use a rag to wipe out all of the excess oil from it.
Now a little bit about lubricants. There are several types of them on the market, in fact there are lots of them. The main difference is the thickness of the lubricant itself. Some tend to be more like olive oil, others are like sap. Some of them attract more dust and dirt, and if the lube is thicker, it can create clots. The choice depends mostly on the king of environment you ride in. Dusty, mountain like – lighter lube, to prevent clots. City like – you can go for the harder stuff.