On: Do you make these 5 database design mistakes?

[cleaver]

Agreed. And certainly better than creating bad indices.

[glhaynes → cleaver]

I wonder: are there any good rules-of-thumb for when to go ahead and add indices at table-creation time?

[BrentOzar → glhaynes]

Add an index to support foreign key relationships. If you've got a SalesHeader table and a SalesDetail table, you want an index on SalesDetail.SalesHeaderID, especially if you allow cascading deletes from the SalesHeader table.

[emmett → glhaynes]

Projected usage. It can be done, but you have to be an expert basically.