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The soil of indoor plants is likely to dry up through evaporation, particularly where there is central heating or if they stand in sunshine. Plastic pots conserve moisture better than porous clay ones. Standing one pot within another is a help; and plants grouped together protect each other from evaporation. Pots standing inside other containers can be surrounded by moss or peat which is kept damp - another way to reduce moisture-loss and keep the leaves fresh with the aid of the water-vapour that will be given off.
Evaporation from the exposed surface of the soil can be reduced if this is covered with gravel, stone chippings (from an aquarium shop), peat or moss. Putting a transparent cover over the whole plant (even a plastic bag) is a useful temporary measure, particularly when going on holiday. Be sure the bag is fastened tightly to the pot, but not touching the leaves - use twigs to keep it away from them.
All but furry-leaved plants like to have their foliage sprayed (use a special trigger-spray which gives a fine mist, or else a clean squeezy bottle of the kind used for detergents). This not only helps to keep a slightly moist 'micro-climate' around the plant, but drips on to the soil. It makes sense to take steps like these to reduce water-loss - not only for the sake of the plants, but to reduce your work in watering them.