I normally just give away Hero/Elite packs by using a random list generator. I put everyone with 3-5 rounds in it and just draw it so everyone has a fair chance. Then I put the people who did get one in a Google doc as well as people who didn't. If they got one they don't get added the next week, if they didn't they get added a second time the next week.

Legendary packs I let everyone vote for who did the best. Of course I include myself in all of this so I don't get a pack every week. People tend to get mad about the leader always getting a pack.