In doing some research for a potential project, I was exploring what kinds of eAdvocacy options were available to plug in to CiviCRM. Many of the big commercial eAdvocacy tools have big commercial price tags to go with them (and don't integrate directly with CiviCRM besides).
Having just had to import data that included a membership number for a client, rather than make a new field for the number it made sense to use the Contact ID, given that this is already a consecutive number and unique to each contact. The problem was that over the years there were big gaps in the consecutive numbers.
While the following is pretty straightforward, I've seen this issue brought up a number of times on the forum so I've written up what we did so that others may find their solution here! I hope its useful: