+1 weighted carries and deads. I also rip my core to pieces with planks on a ball, 90 seconds on 30 seconds off. Repeat as often as you can until you cry. Do 2-3 sets of these, with 5 minutes in between for rest. Yes, it's going to hurt. If it doesn't, do 120s on, 15 off, or some other variable.
And also, incline weighted situps work well.
And there's always crunches.. :P
Mt Snow, VT - May 5, 2012 - Completed!
Gunstock, NH - June 1, 2013 - Completed!
...i remain, he who remains to be...