Reclaimed barn wood is strong and durable, and furniture made from this material lasts a long time if you take care of it properly. Reclaimed wood can often look like it is difficult to take care of, but it's not if you treat it carefully. Key to ensuring it stays in good shape is giving it a protective coat, and then cleaning it like you might clean other wood furniture.
Apply a Protective Coat
Even if you don't paint or stain the furniture, you want it to have a protective coating of sealant or lacquer. This will stop much of the damage you would normally see from plates and cups. You want a coating that is waterproof and hopefully stainproof, or at least one that slows any transfer of staining materials into the wood so that you have time to get a towel to mop it up. If you buy reclaimed wood furniture, this coating will most likely already have been applied, but double-check anyway. If you buy wood to turn into furniture yourself, remember to coat it all before using it.
Keep It Away From Sun and Moisture
Even when coated, though, the furniture can fade when exposed to sunlight or sustain slight stains and rings when exposed to moisture. Both sun and moisture can also warp the furniture if the exposure goes on for long enough. Keep the furniture out of direct sunlight, and use place-mats and coasters when placing food or drink on tables made from reclaimed wood.
Clean and Hand Dry It Gently
Cleaning the furniture, assuming it's been coated, is easy. Get a non-ammonia-based cleaner that is mild and gently wipe down the furniture. Do small portions at a time because you want to wipe away the cleaner and hand-dry the furniture instead of letting the cleaner evaporate. You can find non-ammonia-based dish detergents at many stores, and a little of this plus water should be enough to get rid of dirt.
Dusting the furniture often is a good idea, simply because the shiny coat can make the dust stand out. All you need for that is a dust cloth; don't use sprays that are supposed to make dust lift up as those could harm the coating.
The place that manufactured the furniture may have more specific cleaning and care instructions for the pieces you're buying. As long as you care for the furniture properly, it should last for quite a while.