On the rare cold nights in winter here, when our humidity actually drops below 30% or so, I hang wet towels around the house. I keep adding towels until the windows start to fog up, and that's when I know I've got the humidity up there high enough for my orchids (which are hiding in the house from the cold temperatures). Do your laundry at night, and hang the clothes and towels around to air dry. Run your dishwasher at night and leave the door open when it finishes so the moisture escapes into the room. Don't run your exhaust fans when you're cooking or showering. Again, this keeps moist air in the house.
I learned all those things when we lived in Utah, high desert, very dry. In summer we cooled the house with a swamp cooler aka "evaporative cooler" so that helped with indoor humidity a lot. But in winter the dry air is brutal (I just got back from visiting my kids there). The wet towels are the most effective thing, if you have the patience to do that.
Humidity trays under your plants are also a great way to raise the humidity right around the plants (although more humid air is good for you, too). A plastic or metal tray with pebbles in it (so the bottoms of the pots are held up out of the water, and then kept full of water are a tried and true method. They don't use a whole bunch of electricity to run, either. Surface area is key - the bigger the tray, (and the more wet pebbles) the more humidity.
