Great advice from our forum:
Another thing I've learnt thorugh experience is to ALWAYS have an Autonumber field on all your workspaces. Even if it's something you just put at the bottom of the form. This makes is so much easier to import and match data with exact accuracy later. While you can match on multiple values - you still might have two people with the same name in your contact list.
This is simple - you can add it any time you want - even after records are created - just go to setup and add a single auto-number filed. It will prefex it with two letter - you can see all over the workspaces we've changed these to a two letter abbreviation for whatever your working on - "CN" or "CT" or whatever you want. Makes it easy to identify (doesn't have to be globally unique).
Thanks Brian Roepke for that tip - it stemmed from a discussion on importing, but is useful in other areas as well.