How do I stand on my skateboard without it wobbling from side to side.?

Hi, I'm a tall, heavy built skater (I play in the foward pack for rugby) and I wear size 11 shoes, so I have rather large feet. Anywho, I have a reasonably good skateboard, so I'm uming that the wobbling is not due to the actual board, but my own clumsiness and lack of balance. I just wanted to know what I can do to practise not having my board lean from side to side when I skate.