Here's what it looks like:
With Some Excel documents I've had to authenticate more than 3 times!
It turns out that this is a client-side fix in most cases, due to the fact that for security reasons we don't want to automatically wander through internet content and downloads. Of course in this case we know that SharePoint should be trusted as an extension of our intranet into the Internet space. So that's how you fix it: you tell the browser that SharePoint should be trusted to receive automatic logons by adding it as a Trusted or Intranet Site. However, the fix is slightly different depending on which option you choose.
First, the Intranet Fix:
Open the SharePoint site first!
Click Advanced to manually define Intranet Sites
If you were viewing your SharePoint site already it should be listed in the "Add this website to the zone" dialog box. If not, you'll need to type it and then click Add. Internet-facing SharePoint sites should be secured with https, but if yours isn't you'll need to weaken security by removing the "Require server verification (https:)" checkmark.
Voila! You're done and should only need to log in once to your SharePoint site from now on
But wait a minute, some of you are saying, shouldn't I assign this as a Trusted Internet site instead of an intranet site? I mean, it is on the Internet... To which I say, "Yes!"... but it is slightly more complicated.
Same first steps, but this time choose trusted sites instead of intranet
Open your SharePoint Site first
Open internet options
Navigate to the Security tab, choose Trusted sites, then click Sites
Your site should be present in the address bar, click add and close
With Trusted Sites still selected choose Custom Level
At the bottom of the list of custom settings, change User Authentication to "Automatic logon with current user name and password"
Voila! You now have the added protection of Trusted Internet sites with the ease of authentication found in intranet sites.