Any creative or elegant ways other then telling then a email campaign (like the following): Hey {NAME}, we would love to send you stickers but dont have your mailing address. Would you mind filling out this form with your data? Of course the wording would be done by a copywriter instead of me. But are there any ways other than email campaigns
Do you have a monthly/weekly newsletter that you send to everyone?
If so just write an article there about: Check that your mailing address is correct in your profile now for a chance to receive free gifts: <link>.<p>Same suggestion if you have a blog, etc.