For example, maybe losing one sweepstakes means that you have more time and motivation to enter more often, helping you to win more than if you had settled for the original prize.
Or take an example of someone who trips on the sidewalk and twists her ankle. An unlucky person would probably take that as an example of yet another piece of bad luck. A lucky person would probably say that it shows how lucky she is that she didn't break her ankle or worse.
By looking at the positive aspects of even bad luck, you can continue to have a lucky attitude.