"All you have to do is find the .dmg file hosted online somewhere. Sure, you can't readily download premium apps without paying for them, from the App Store, but that's never stopped files from ending up on pirate websites before. Once you've found the app, all you have to do is install it as you would any other application and then copy over 3 files (and/or folders) from any legitimate download that you've made in the App Store -- even if it's a free download (Twitter, for instance)."
The piracy can cut both ways: legit apps can be posted on shady sites for people to grab without paying for them. And phony apps can be posted to the store, look alike Angry Birds for instance, that are Trojan horses for malware. This is why we can't have nice things.