How much water does the plant need?  This is probably one of the first questions you ask a salesclerk.  And the stock answer seems to be, "Water when it's dry."  Generally, that's good advice--but some plants should never be allowed to dry out completely, and others shouldn't be watered until the soil is bone-dry.  A bit of experience and a drooping plant or two will teach you quickly when it's time to water again.

Varying conditions determine how often you should water your houseplants.  For instance, if the air in your home is dry, you'll have to water more frequently than if the air is humid.  Plants in small pots need water more often than those in large pots.  A plant in bloom needs extra water.

There is one golden rule to follow: when you water, be thorough.  Supply enough moisture to soak the soil all the way to the bottom of the pot and into the saucer.  But don't let the plant stand in this excess water

Thorough watering once a week is better than shallow watering every other day.  More plants perish from overwatering than from underwatering.  Excessively wet soil crowds out the oxygen needed by the roots, and the roots can't support the leaves if they don't have oxygen.  Even plants needing a lot of water can be killed if they are kept soggy.  If in doubt, don't is a good rule to follow until you've established a successful watering schedule.

Checking plants twice a week is a good habit to adopt.  Check early in the week, and water the plants that are dry--coleus, pickaback, hemigraphis, and small specimens will probably need it.  Water the rest once a week or so (or more if your home is very dry).  Cactus and other succulents may go ten to 14 days between waterings.  Every month or two, water your plants by immersion to be sure that all of the soil gets watered.  To immerse a plant, fill a pail or dishpan with water deep enough to come just under the rim of the pot.  Set the pot under water and leave it until bubbling stops.  The time this takes depends on the size of the soil mass and how dry it is.  Remove the pot from water, and set it in the sink to drain completely.  Meanwhile, clean the foliage.  Don't go away and leave a plant immersed for longer than necessary.  Too much water over a long period of time prevents oxygen from getting to the roots.

Containers should also be considered when watering plants.  A clay pot presents the fewest dangers because it absorbs moisture from the soil, lessening the danger of waterlogging at the bottom of the pot.  When the surface of the soil in a clay pot feels dry, you can safely assume that the soil in the bottom of the pot is dry and that the plant needs water.

Most plastic pots have drainage holes in the bottom so excess water can escape.  But, because the plastic isn't porous, the water held by the soil doesn't evaporate as fast as it does from a clay pot.  The soil surface can feel dry while the soil at the bottom of the pot is still wet.  Water plants in plastic pots less often.

Ceramic containers are the riskiest to use.  They usually have solid bottoms, so there's no way excess water can escape.  Water carefully--just enough to moisten the roots--never let water run over the rim of the pot.  Water more often but not as thoroughly.

An easy way to water plants, especially when you want to use ceramic containers, is a technique called double-potting.  Simply take a plant in a clay pot and set it inside a ceramic pot two sizes larger.  Line the space between pots with sphagnum moss.  Keep the moss moist by soaking it once a week.  The moisture will seep through the inner clay pot wall to provide even moisture.

If you have plants potted in clay or plastic containers and set inside ceramic containers, be sure to lift out the inner pot after watering and empty the water from the jardiniere.

Adding humidity.
Most plants need more humidity than is normally found in the home--especially during the dry, heating season.  The colder the air outside, the drier the air inside, and dry air can be disastrous for plants accustomed to tropical conditions.  Most plants require 30 to 40 percent humidity; your home may have less than ten percent humidity when it's extremely cold outdoors.

More frequent watering will help plants somewhat, although the dry air will still cause the leaves to transpire (lose water they've absorbed) at a faster rate.  The trick is to supply more water to the leaves.  Humidifying your entire house is ideal, but you can increase air moisture around your plants in other ways.

Frequent misting will provide some relief; you might run a cool vaporizer in rooms where plants are kept.  Or hand-mist in the morning, so plants will dry off and not stand damp overnight.

Plants that require constant humidity, such as ferns, can be placed on a moistened tray of pebbles, which will increase humidity as the water evaporates.  Keep your plants in a close group, rather than scattered about, to help ease their need for humidity.  Set pans of water near heat registers, and if you don't mind company when you shower, take your plants into the bathroom with you so they can share the steam.