Only being seeded on sends of >=500 might not be possible. What I do is just always make sure I include the designated seed list (of which I'm a member) on the smart list for the email program. Since most of the time I'm just cloning these programs and changing things, I don't really have to worry about adding it each time.
The few times I start from scratch, I've got into the habit of making sure I always add my seed list and master suppression lists first.