Many times in an Extranet scenario I see that Clients are having to enter domain\username in the username box, although this isn't a particular problem if often causes a few user error issues which could be easily avoided.

1. Locate your externally facing website in IIS

2. Choose Properties on this web site

3. Choose the Directory Security tab

4. Choose Edit.. under the Authentication and access control heading

5. In the Default domain box enter the domain that users will use and click OK twice

Done! Your users will no longer have to type domain\username just username instead