I know there are thousand posts about this, but for easy referencing in the context of the other tasks I'm doing (for instance when building the Retail CRT solution) you need to generate a .snk file to sign your dev dlls.
You do this by running sn -k strongnamekey.snk.
The path in which you run the command is different between the VS versions, but you should get there by running the Visual Studio Command Prompt (2010) or VS2013 Native Tools Command Prompt.
Now, after you have generated your .snk file, you can choose it to sign your CRT assemblies.
Once that is done, you will have to copy the new dev dlls to the CRT Package folder, which by default is c:\Program Files (x86)\Microsoft Dynamics AX\60\Retail Server\Package\bin\. You are also getting your .pdb files, so you can debug the CRT afterwards. And, finally, but not last, you have to replace in the .config files the new version / tokens for the dlls you've replaced.