If I start with Mongodb on a single server, but foresee that I need to scale out, should I configure it in any specific way? I guess I am looking for tips from people who have gone down this route.
No, Replica Sets and Sharding (the two methods of scaling and redundancy) do not require any special treatment to occur on the solo db so that you may later add them in. In fact, adding replication is reasonably painless and straight-forward. (Sharding does require a little more effort, but is still pretty easy.)