There are various ways to humidify the air in a house. Possibly the most obvious is to use a humidifier; make sure yours is of the right capacity for the size of the room it’s in. Many if not most furnaces can be outfitted with a device that remoistens the air after it’s been heated, and this has the advantage of humidifying the entire house. People living in apartment buildings, however, might not have this option. Indoor fountains are also a good way to introduce moisture into the air, especially the ones that create mist.
There are also low-tech options and they can be surprisingly effective. A deep saucer filled with stones and then topped up with water is a classic, tried and true method. It is so effective I find I have to refill mine (a 10-inch saucer) every two or three days. For maximum effect you would ideally have one under each plant. I place mine on a stand over the air vent nearest the plants. Failing all else, shallow pans of water will suffice, but do make sure that pets and children (and adults for that matter) don’t make it a hassle.
Cooking helps, believe it or not, if it’s done regularly. I always recommend that people cook for themselves rather than eating out. It’s usually healthier and cheaper, it’s a useful skill, and it helps to humidify and warm the house. A lot of people say they’re too busy, or stressed from work when they come home, to cook, but it can be quite calming and therapeutic if you give it a chance. It doesn’t mean you have to become a pretentious foodie and it doesn’t have to be a gourmet production.
Speaking of cooking, kitchens and bathrooms tend to be the most humid rooms in the house. If they have enough light and space, they’re actually pretty good places to keep houseplants – if you don’t mind having them there.
Smaller plants can be kept in or under large transparent jars or containers. Remember to lift the covers off for a few moments every couple days or so in order to let fresh air circulate in. Large plastic soda/pop bottles with the bottoms cut off make good covers, especially since you can leave the cap off to provide ventilation. Beware of scorching if you leave these in direct sun; the covers (especially glass ones) can focus sunlight to a damaging degree. Wardian cases, cloches, and terraria are great ways to deal with low humidity.
Some plants, especially citrus and many tropicals, appreciate being sprayed or misted with water on a daily (or even twice daily) basis while the air is dry. If you occasionally do this to the point of the water beading and running off, it has the benefit of washing off any dust that settles on the plant. Also, as the water evaporates it increases the humidity in some small way. However, you have to balance misting against two things. One is the mess that it potentially creates, especially if you do it to the point of dripping; watch out if you have nice hardwood or parquet or carpeted floors. The other, and more important in my eyes, is that if the water sits on the leaves for too long, it can encourage fungal diseases to take hold.
Humidity for houseplants can really be a balancing act. You want it to a level so that plants will survive comfortably, but not so high that it encourages diseases or damages your house. Getting it high enough to damage the house if you use central heating, however, is unlikely.
I’ve never lived in a house that didn’t use gas furnace central heating, so I don’t know how other heating systems affect humidity in winter. If anyone reading this has some such experience with other heating methods, I wouldn’t mind some insight.