Don't overwater. This is the most common mistake terrarium keepers make, according to experienced gardeners. Terrariums, by their glass-enclosed nature, create miniature ecosystems that are self-irrigating. Do check your soil weekly, with a finger, to assess whether it's dry or moist. "But that doesn't mean water it weekly," said horticulturist and designer Mary Beth Gullickson of Bachman's. A terrarium should never have standing water at its base. If needles or leaves appear dry, it's better to mist than to water, she said. And if you have a cloche (bell jar terrarium with a lid), lift the lid periodically to air out plants and release condensation.

Don't just drop plants into place. "You need to dig a small hole to receive the plant," said Tovah Martin, author of "The New Terrarium." And once the plant is planted, firm the soil around it, making sure all the roots are buried. An exception is orchids, which fare better when surrounded with bark than with soil.

Don't neglect pruning. "A terrarium will always outgrow itself eventually, if it's healthy," Gullickson said. "Don't be afraid to give plants a haircut. You're actually doing them a favor."

Do put horticultural charcoal at the base of your terrarium to filter the water and sweeten the soil.

Do try terrarium-friendly plants, including ferns, mosses, baby tears, selaginella, miniature African violets and small-leaf begonias.

Do seek dwarf species and small-scale plants. A baby plant that looks small and cute on the nursery shelf may outgrow your terrarium quickly, cautioned Pamela Larson Frink, who teaches a community-education class on terrariums. "Ask people who work there how big it will get."

Do make sure plants are healthy before planting them in a terrarium. Gullickson advises looking for a healthy root system, with firm, white roots.

Do start with a wide-mouth container, if you're new to terrariums. It's easier to tend a terrarium when you can fit both hands inside. Once you become more proficient, you can experiment with smaller containers, using chopsticks, pencils or terrarium tools, Gullickson said.