The least horrific way is putting it in a paper bag and hitting it against something pretty hard...
Other ways I've seen/heard of people doing it is holding the mouse by the tail and hitting it against something, or putting a nail in its head.
I'm so happy I was able to get my snake to take f/t.

PS I'm wanting a couple of Tomato frogs. Do you think that the temporary setup I've got for my baby pixie would be adequate for a couple?