I'm an IntelliJ user, so I have the SBT idea plugin permanently in my global SBT plugins. This is fine, until I start working across versions of SBT, and particularly with milestone builds of unreleased SBT versions, because in that case, the version of the idea plugin that I usually use with other versions of SBT may not be available. Hence I need to have per SBT version global plugins. I think this might be easier to do in future, but for now, here's a way to do it:

libraryDependencies <++= (sbtBinaryVersion in update, scalaBinaryVersion in update) { (sbtV, scalaV) =>
  sbtV match {
    case sbt013 if sbt013.startsWith("0.13.") => Seq(
      Defaults.sbtPluginExtra("com.github.mpeltonen" % "sbt-idea" % "1.5.0-SNAPSHOT", sbtV, scalaV)
    )
    case _ => Seq(
      Defaults.sbtPluginExtra("com.github.mpeltonen" % "sbt-idea" % "1.4.0", sbtV, scalaV)
    )
  }
}