Actually for any exercise, try sucking in the gut and tensing the abs. But especially for biceps exercises and lighter back exercises, it's been giving me some great workouts over the last few weeks. Just started trying this out right before the New Year.
I know a lot of you probably already do this, but it's not something that I've focused on. The main benefits are that you are essentially training your waist and you are forced to perform exercises in a strict manner that gives you an amazing pump. The secondary benefit is that the limited breathing ups the intensity, kind of like autoerotic asphyxiation. I just got back from a light leg workout and I was dripping buckets of sweat. (For me, sweating during my workouts is a big deal).
So, if you're not already doing this and you want to try something, give it a shot.