Ah the crazy monkey we drill this alot, heres a gd way to practise.
Stand with your rear foot against wall so you got nowehere to go, the aim is to stand still no moving around at first. Get your sparring partner to stand in front of you still as well and start firing off punches. Slow at first then build up.
Put your hands just above your forehead, palms facing down and try and take as many shoots as you can on your elbows. When he goes to the body rather than dropping your hands just bend your knees and your elbows will come down to meet those punches as well.
Tito Vs Chuck is great example, just before tito gets knocked out (dont practise it with chuck) watch what he is doin with his hands and elbows.
We had a guy from our club use this in a fight, the other dude threw a couple of combinations and rocked him so he resorted to crazy monkey, took a punch on his elbow and the guy throwing the punch popped his shoulder with the impact! Good Times.