I hate spending so much money on those darn plug-ins but they are a must in my house! I found a way to be frugal about it!
Take your plugin and take the lid off as well as the cigarette looking thing.
Take your favorite detergent, fabric softner, or even aroma oil and fill about 1/3 of the way.
Fill the rest with water and shake!
Put lid and cig thingy back on! VOILA!
Plug it in!