What’s the easiest way to raise humidity for plants that need a humid environment?

Fill saucers with an inch or more of pebbles and set the potted plants on top. Keep the saucers filled with water just below the top surface of the pebbles. Other ways to increase humidity are to spray plants lightly with water once or twice a day, or set pans of water among them or on nearby radiators. Another fine source of humidity is your clothes dryer.  If you can figure out a way to exhaust the moisture from it into the room where you grow those plants that most need humidity, they will have all they need. And, of course, room humidifiers will always do the trick.