Pick yourself up and move on. Persistence is the key to success. I've read that a good way to maintain a firm stomach is through respiration exercises while doing chores or simply standing around. The process of breathing does use muscles along the rib cage and abdomen, but I'm not aware of breathing exercises that strengthen those muscles. Check a library for books or videos about yoga, which emphasizes controlled breathing.
More work equals better results.
You can work the abdominals every day. The traditional crunch is the safest way to tone abdominal muscles. Lie on your back on the floor, feet flat and knees bent. Put your hands behind your head, elbows out. Slowly roll up, contracting your abs and keeping the small of your back flat on the floor.