Salts are a good way to do it. Another useful tool is a chilled mirror dewpoint sensor. Commercial ones are expensive, but it turns out building one is quite easy. It's nothing more than a mirror mounted on a peltier device, with an LED and phototransistor (or diode) to sense when the mirror fogs up. With some feedback, you can hold the mirror just at the dewpoint. If you have the dewpoint and the ambient, you can calculate the RH. Same calculation works for a wet bulb/dry bulb thermometer. I've done all these and can say, like the man with two watches, you're never quite sure what the RH is, since the methods won't agree precisely.