Ah, lower abs, my bad. Here is a good one: Try laying on a straight bench with your knees bent over the edge at a 90 with your feet flat on the floor and your back flat against the bench. Cross your arms over your chest and do a full situp. As you situp tense your abs and breath out hard like your taking a big dump (although actually sharting is frowned upon in most gyms). Make sure to touch your shoulder blades as you lay back. Repeat these as fast as you can for 25 reps. Rest for 30 seconds in between sets and do a total of 3 sets. At the end of your 3rd set immediately raise your legs up to a 45 angle towards the ceiling and flutter kick (kicks like when you swim the crawl) for 1-2 minutes straight with your knees locked (the longer the better). Adjust the angle of you legs until you feel it burn in your lower abs. Don't mind that you may looks a little fruity while doing the kicks, this final burn on your abs is what will really help your endurance.