When I was a member of the Mage Knight forum I belonged to a clan there. Our membership was purley based on clan member referrals. We had a great, active clan because of it. What we looked for was active, intellegent posters who conducted themselves well. By the time the forum fell from it's hay day glory, our private clan forum had the 4th largest thead count out of any forum on the site! That's the type of thing I want for the Vets clan. I want active members who have proven themselves a part of the community.
To begin with, I think having a join date or a minimum post count is a fine way to go. I'm thinking the join date should be before my "LordBarronmore" account join date to even be considered. That way I KNOW you were here by an approprate date considering I made that account the day I left.
Second, minimum post count I think should be between 500 and 1000. Maybe I've been off the site to long and that's low, but I think that the minimum post count should be high enough to discourage spammers but low enough to allow those that have proven themselves as active community members to have a chance to join the clan.
After the initial recruting stage, membership will only be granted by minimum post count and member recommendation.