http://docs.nuget.org/docs/reference/versioning
You want the section labeled:
Prerelease Versions
//Quote//
Additionally, prerelease versions of your API can be denoted by appending an arbitrary string to the Patch number separated by a dash. For example:
1.0.1-alpha
1.0.1-beta
1.0.1-Fizzleshnizzle
Note that the actual string applied doesn't matter. If there's a string there, it's a prerelease version.
When you’re ready to release, just remove the dash and the string and that version is considered “higher” than all the prerelease versions. For example, the stable version 1.0.1 is larger than 1.0.1-rc
//End Quote
and then the "how to get it" command-line argument
Install-Package CoolStuff -IncludePrerelease