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private static void preOrderTraversal(BinaryTreeNode root) {
		Stack<BinaryTreeNode> s = new Stack<BinaryTreeNode>();
		while (true) {
			while (root != null) {
				System.out.println(root.getData());
				s.push(root);
				root = root.getLeft();
			}
			if (s.isEmpty())
				return;

			root = (BinaryTreeNode) s.pop();
			root = root.getRight();
		}
	}

Algo :
1) Push root into Stack
2) Now iterate steps 3,4,5 with condition - while(stack is not empty)
3) Pop a node, print it
4) if node->right is not NULL, push it
5) if node->left is not NULL, push it

Full working code here :
awesomecoder.blogspot.com/2012/08/preorder-traversal-of-bst-iteratively.html

The code goes like this:

void preorder(tree *T)
{ Stack S;
while(T!=NULL)
{push(&S, T);
T=T->left;
}
while(!empty(&S)
{
T=pop(&S);
printf("%d",T->data);
T=T->right;
while(T!=NULL)
{
push(&S, T);
T=T->left;
}
}
}

You can understand the concept by watching this video

Iterative Preorder tree traversal
youtube.com/watch?v=uPTCbdHSFg4

Iterative postorder tree traversal
youtube.com/watch?v=hv-mJUs5mvU

Iterative inorder tree traversal
youtube.com/watch?v=50v1sJkjxoc

It is funny they called this a pre-order traversal, for this type of traversal has a name which is called a level order traversal. It can also be implemented using a queue, which is actually a preferable implementation when doing a level order traversal on a tree.