A very effective technique against inexperienced boxers. Sometimes even works against experienced boxers, but I doubt it would be so effective in MMA. The reason this works so well in boxing is because of the ability we have to rest in the clinch with no fear of being taken down. I'm not a MMA fighter, but I doubt that resting like that in the clinch is very common.
Here is the technique:
Imagine you have your opponents head resting on your right shoulder and you have overhooked his right arm with your left arm. In one short quick movement you ram your shoulder upwards QUICKLY, and then step back slightly and hit him with your left hook. When done correctly his head will be popped up and jaw exposed with no cover on that side because your left arm was blocking his right side. You have a beautifull shot. Once they are familiar with this you can be sure you won't get the opportunity again - so make sure when you do it you really slam that hook home.