I've recently added a couple of exercises to my routine.
As I've mentioned before my work-out area is unheated in the winter; these can be done in the cold and do seem to make a difference.
Ab Contractions:
From a standing stance, hunch your shoulders slightly, breath out, then pull your abs in, concentrated on the pull of the abs. I'm doing three sets of sixty. These are relatively easy and make a good exercise between sets of other exercised. These do seem to make a difference.
"Horizontal Pullups":
From Robert dos Memidios' "Cardio Strength Training."
Place a bar across the safety bars of a cage (you could also do this with a bar suspended by chains from an overhead anchor point)).
"Hanging underneath a bar with your legs straight and core tight, pull yourself up until your chest touches the bar..."
Doing this with your feet flat on the floor and your knees bent is good for warm up, and actually fairly easy. If your are masochistic and want a better workout, put your feet on a bench and try to touch your chest to the bar.