No strict norm but in general:
- One topic on The Request Forum clearly listing all your requests first by console and then alphabetically, named something like (in your case): Requests by TheIndyKid or some Xbox360 requests
- Asking politely (and saying thank you both in advance and again once the request is filled if you want more requests filled in he future).

- Then just wait and hope that someone who has a scanner has the requested cover and time to scan it properly following all scanning guidelines, and then that one of the artists has time to clean the raw.
Unfortunately it has been getting quiet here so request can take long time to be fulfilled.