I want to publish my pictures with a CC-BY-NC license but I could not find a tutorial how store this information with my JPEG metadata.
Is there a best practice? Do services like Flickr, Google+ or Facebook read this information?
As @coneslayer notes, Creative Commons does indeed provide guidance for XMP information.
The main thing is to set xmpRights:UsageTerms to This work is licensed to the public under the Creative Commons Attribution-ShareAlike license http://creativecommons.org/licenses/bysa/2.0/ verify at http://example.com/pdfmetadata.html
That "verify" link is optional and should be a web site explaining the licensing in more detail. If you have such a web page, you should also set xmpRights:WebStatement to that URL.
They also recommend duplicating xmpRights:UsageTerms into dc:rights, because some people might use that tag instead.
You also can and should set XMP properties using Creative Commons' own schema, which defines cc:license, cc:morePermissions (for possible other allowed uses), cc:attributionURL, and cc:attributionName.
If you are using Photoshop you can go to File > File Info - This will allow you to include copyright data in the JPEG Metadata.
I am not aware of how services like Flickr, Google or Facebook handle metadata so I cannot comment there.
I use this little script, which you can easily adapt. It need Python2.6 (does not work with Python3 because FWIK the pyexiv2 library has not a Python3 version). This come pre-installed with practically all linux distributions; you may need to add the package
I think is more or less self-explicative: if you need to change/add Xmp tags, you simply do