With the windows closed on a cool day, it's pretty easy to raise humidity with a cool mist humidifier,
like this one and/or
mist fan. If you've got a big room (or greenhouse/conservatory), you can get big foggers that are really effective.
Just remember that with more humidity, you should have more air movement (fans) or you'll have trouble with bacterial/fungal rot.
If it's not ridiculously hot and dry outside, I can manage to keep the humidity level pretty much anywhere I want it, as the EvaMist unit has a hygrostat (like a thermostat for humidity).
I can even monitor it from work as I installed a
network data centre monitoring device in the growing room, and if necessary, go home and do something if it drops to low. The system can even be set to email you if things go out of range (or you can use something like Nagios to monitor it and alert you).