What are the best practice for implementing authentication for REST apis?
Using BASIC auth + SSL or something like http://tools.ietf.org/html/draft-hammer-http-token
You should look into Security Token Service or STS.
Security Token Service
Check out these links for more information:
Off-the-shelf Security Token Service (STS) that uses ASP.NET membership provider?
http://msdn.microsoft.com/en-us/library/ee517259.aspx