One of the things the physio said to me to do is to always have good posture and try to suck your tummy in when you walk. Tip your pelvis forwards slightly have your shoulders down and back imagine a string holding your head up high and imagine sucking in your belly button to your spine. When you walk anywhere do this every time. It will help strengthen your whole core not just your tum. It's uncomfortable and felt a little unnatural to me at the begining and I couldn't hold my tummy in for long when I started, but now it's kinda second nature and my tummy is getting flatter and stronger, and I have maybe a 1-1/2cm gap now (it was about 3-4cm) and I don't do sit ups or any other exercises!!