Take an old cookie sheet, spread some gravel on it and set the plants on top of the gravel. Then, pour some water over the gravel in the cookie sheet. So, we never have the plants actually sitting in water. They're sitting above the gravel. The water will evaporate and increase the humidity locally.
Temperature seems to be another important issue.
My meyer's lemon sits in a bay window that's very drafty and it doesn't seem to mind, but other plants really suffer. Again, these are tropical to subtropical species. They need warm temperatures. They like the temperatures in the 70-72 degree range during the day when the lights are on. At night, you can set the temperature down.