[simplyesl]

I shake my head and emphasize the negative part of the answer (not/can't etc) for negative answers.
I think flashcards are great (especially since most of my drawings look nothing like the real object), but I like to use the whiteboard and slowly reveal the language.
If the students are being quite I will hold my hand up to my ear and ask "what?" This gets the students to say it louder, as well as getting the quieter ones to join in, too. If they aren't loud enough after you've said 'what?' the first time, say it until they are loud enough/have all the students speaking.