Breathing with your diaphragm (from the belly)
Pelvic tilts (standing, bent over with flat back, on all fours, on your back)
Transverse ab work seated (pulling abs in, held for 30 seconds, lots of reps contracting)
Most of these you can do while sitting at a desk, standing or driving, so you can plug them into your day wherever.
In general, you don't want to do crunches, Pilates 100s or anything that makes your abs or belly bulge (including...