In a comment to the post below, DarkoV recalls the glory days of "riding at a 45 degree angle, on training wheels." Training wheels were never an element in my past, but when it came to my daughters I indulged them. I even attempted actual "training" with said wheels, by elevating their angle. After a few minutes of watching my daughter honing her 45 degree technique, I figured I'd better avoid the chiropracter's bill and get those wheels back on the ground.
Fortunately, there's a better way to get kids used to the balance of a bicycle. Lower the seat so their feet can rest securely on the ground, then remove the pedals. The kid pushes the bicycle, like she would a scooter, and very quickly gets a sense of its balance. My girls are cautious by nature, so it took a couple of afternoons of this before they got the hang of it. But I've seen other kids master it within an hour.