Over watering is the most common problem with terrariums, but on the flip side they are living and therefore require enough water.
For open terrariums, as long as no water is pooling at the bottom of the container then your terrarium is either doing fine or needs some water. Very quickly the leaves of your plants will droop if they are not receiving enough water.
If the soil is dry, add some water. Use the 'spray' function of the water spray bottle to dampen the soil, this may take quite a few sprays. The 'stream' function may seem more efficient, but water tends to run straight through the soil profile without sufficiently dampening the soil where the plant's roots are, therefore we recommend spraying the soil around the base of each plant.
For enclosed terrariums, you don't need to add water very often at all (approximately monthly). As a general rule, if you can see condensation in your enclosed terrarium on a warm day, there is no need to add more water. Don't allow water to sit at the bottom of the terrarium for extended periods of time as this may cause plant roots to rot. You can use paper towel to soak up excess water. Otherwise remove the lid and place in a brighter area for a few hours (but not in direct sunlight).
Only ever use de mineralised water (available from supermarkets and hardware stores) or rainwater as tap water contains salts that can build up inside the terrarium causing damage to your plants. After watering, allow the leaves of the plants to dry before replacing the lid. For enclosed terrariums, remove the lid for a day once a fortnight to allow the terrarium to air out.
A handy tip for open terrariums is to cover their opening with glad wrap if you are going on holiday or don't have time to water it for a period of time.
For succulent terrariums please err on the side of under watering. Only once every ten days is sufficient and only one teaspoon or two sprays is adequate.