"Oh dammit", you think while you break out in sweat, "now I wish I was still human..." Your mind focuses completely on your wish and your wings melt back into your body, your tail shrinks away just like your little horns, and your teeth and tongue regain their normal shape. Surprisingly, it didn't take more than a moment...
"Yes, everything is fine, thank you", you say as the maid pokes her head around the door, "I was just taking a bath." She nods. "Very well ma'am...I'll leave you to it then." She closes the door, and as soon as she's gone you try wishing for your demoness form...within a moment, all your previous features return, although you take note not to grow wings in the bath as it is a rather cramped space. Since you are able to invoke both forms at will, you start wondering how many people you could give a nasty surprise this way.