Cold dry air

[unclrob]

When I lived up north one trick we had was to put a bucket of water under our heater to help keep the humity up.

[B0WIE]

Good tip. 
I don't have that kind of heater but my personal trick, if a humidifier breaks or I find the humidity is in the emergency zone is I'll run the showers on hot with the most misty setting possible on the head to get the home humidity up for a few hours. That's gotten me through some scares.

[Rockysdad]

Boiling a pot or two of water on the stove also helps bring up humidity.

[John in Charm City]

for electric heat pumps, placing a humidifier in front of the air handler intake will make a huge difference.

[Queequeg]

Each guitar case has a plastic soap dish with holes drilled in the top and a sponge inside.
I have to freshen them about every 2 weeks this time of year.
It means I have to keep the cases lying flat though.
And I run a large room humidifier from November through March, sometimes April.

[Riverbend]

We have a geothermal heat exchanger and, even with the attached whole house humidifier set at 50%, our RH often dips well below that in the very cold weather we're experiencing now in Michigan. My music room, an open-air loft, often shows much lower RH on the hygrometer I have placed near my guitars, all of which are out on stands. I run a room humidifier on days when the RH drops below 40%, hoping to keep that area in the 45% range. I also have an extra evaporative humidifier on the way to help out, especially in extreme weather such as we've had lately. I can hear a difference in my guitars and mandolins when the air gets too dry. I've also used the homemade soap container/sponge case humidifiers for years before we had a whole house humidifier.     

[Pilgrim]

If a quicker dose of humidity is called for, put the guitar on a stand in the bathroom with the door closed.  Take a shower and leave, closing the door behind you.  Come back in a half hour and retrieve the guitar.

[Queequeg]

If a quicker dose of humidity is called for, put the guitar on a stand in the bathroom with the door closed.  Take a shower and leave, closing the door behind you.  Come back in a half hour and retrieve the guitar.

That's a good "one guitar" solution.
 

[BlueBowman]

I have in the past used some of these remedies, but really I'd recommend everyone buy a good humidifier. $150, if you can swing it, is a small price to pay for peace of mind. I've been using my same AirCare for four years now. As long as I clean it once a month and use a bacteriostatic/conditioner, it works a charm.