Cleanest way I've found so far to divide Spray's routing into separate files.

spray_routing.scala

/* Routing trait for mixing in routes */
trait RoutingService extends Directives { this: HttpServiceActor =>
  private val EmptyRoute: Route = { context => Unit }
  def routes: Route = EmptyRoute

  /* Used to concatenate `Route`s together */
  def concatRoutes(superRoutes: Route)(routes: Route): Route = superRoutes match {
    case EmptyRoute => routes
    case _ => superRoutes ~ routes
  }
}

/* First set of routes */
trait Service1 extends RoutingService { this: HttpServiceActor =>
  /* `super.routes` must be passed in for each Service trait */
  override def routes = concatRoutes(super.routes) {
    path("service1") {
      complete {
        "service1"
      }
    }
  }
}

/* Second set of routes */
trait Service2 extends RoutingService { this: HttpServiceActor =>
  override def routes = concatRoutes(super.routes) {
    path("service2") {
      complete {
        "service2"
      }
    }
  }
}

/* Root actor to handle incoming requests. Mixes in service traits to pull in `Route`s */
class RootService extends HttpServiceActor
    with Service1
    with Service2 {
  val combinedRoutes = routes
  def receive = runRoute(combinedRoutes)
}

/* `App` to bind HTTP to the `RootServer` actor */
object Main extends App {
  implicit val system = ActorSystem("system")

  val service = system.actorOf(Props[RootService], "root")

  implicit val timeout = Timeout(5.seconds)
  IO(Http) ? Http.Bind(service, interface = "localhost", port = 8080)
}