The person who said to paint the ball black one side, and white the other is correct. You have to keep the seam upright the whole time. Your also have the bowl at a good pace to get the ball to swing. Without pace, the heavier side of the ball makes to ball rotate and the seam will look scrambled. If you bowl it with pace, and more importantly, backward rotation created by your wrist behind the ball ****ed, the seam will remain upright.
All this will swing the ball, but to get extra nasty swing:
I don't think anybody has mentioned it yet. But another important thing where your point the seam. If you have the seam pointing toward the slips and the shiny side the side of the slips, then the ball with swing more. However it becomes harder to keep the seam upright because your wrist is pointing toward the slips, but your arm isn't. This is why you see even pros like Steve Harmison (who did it last year in the final one-day game against Australia) bowling balls that get caught by slips. They're trying to swing the ball so much that their wrist is dictating where the ball is going, not their arm. It also help - if your bowling out-swing, to keep your arm a little rounded. Ray Lindwall is the prime example of a cricketer keeping his arm rounded.
[Edit - sorry that's how to bowl out-swing]
If you notice, many cricketers don't bowl in-swing because it isn't suiting to their bowling action. You do need to be a little front on. Wasim Akram could swing the ball both ways better than perhaps anybody in cricket history because of his front-on action.
But this is sounding complicated so I'll say this. Start off:
1. Learn how to keep a seam upright. Remember pace and backward rotation - which will come from a ****ed wrist that will have your fingers running down the seam of the ball.
2. While your learning at the start, keep your arm and wrist in line and never forget (perhaps mos importantly) keep your head still!
3. Learn how to properly shine the ball, never shine in the center of a side, just around the seam on one side.
4. If you can't do all that, start pointing the seam in the direction you want the ball to swing.
In fact, enwikpedia explains it all much better than me. It even explains how a ball will reverse over time. Gop to enwikpedia and type in "in-swing". It'll talk about seam positions, keeping it upright, shine etc.
If none of what I said makes sense... go there because it's very concise and easy to understand.
http://en.wikipedia.org/wiki/Main_Page