For iPhones/iPads running iOS 6, you can Limit Ad Tracking for apps by navigating to Settings > General > About > Advertising > Limit Ad Tracking and turning the feature on.

On iOS 7, the option is now also available at Settings > Privacy > Advertising. You can also manage the privacy settings for individual apps on iOS 6 or 7 by navigating to Settings > Privacy and selecting the type of data you would like to manage app settings for.