Here's an SBT build file I work on that I think is pretty clean.

Dependencies.scala

package com.rackspace.dcx.dcorch.build


import sbt._


trait Dependencies {

  private object v {
    val akka = "2.1.4"
    val argonaut = "6.0-RC1"
    val eaio = "3.2"
    val logback = "1.0.4"
    val scalaCheck = "1.10.1"
    val scalaTest = "2.0.M6-SNAP21"
    val scalaZ = "7.0.0"
    val spray = "1.1-M7"
    val sprayJson = "1.2.3" // pinned low to avoid conflict with current Spray
  }

  val akkaActor = "com.typesafe.akka" %% "akka-actor" % v.akka
  val akkaAgent = "com.typesafe.akka" %% "akka-agent" % v.akka
  val akkaSlf4j = "com.typesafe.akka" %% "akka-slf4j" % v.akka
  val argonaut = "io.argonaut" %% "argonaut" % v.argonaut
  val eaio = "com.eaio.uuid" % "uuid" % v.eaio
  val logback = "ch.qos.logback" % "logback-classic" % v.logback
  val scalaCheck = "org.scalacheck" %% "scalacheck" % v.scalaCheck % "test"
  val scalaTest = "org.scalatest" %% "scalatest" % v.scalaTest % "test"
  val scalazConcurrent = "org.scalaz" %% "scalaz-concurrent" % v.scalaZ
  val scalazCore = "org.scalaz" %% "scalaz-core" % v.scalaZ
  val scalazEffect = "org.scalaz" %% "scalaz-effect" % v.scalaZ
  val scalazScalacheck = "org.scalaz" %% "scalaz-scalacheck-binding" % v.scalaZ
  val sprayCan = "io.spray"  % "spray-can" % v.spray
  val sprayClient = "io.spray"  % "spray-client" % v.spray
  val sprayIo = "io.spray"  % "spray-io" % v.spray
  val sprayJson = "io.spray" %%  "spray-json" % v.sprayJson
  val sprayRouting = "io.spray"  % "spray-routing" % v.spray
  val sprayTestkit = "io.spray"  % "spray-testkit" % v.spray % "test"

}

FooBuild.scala

package com.rackspace.dcx.foo.build


import sbt._
import sbt.Keys._


object FooBuild extends Build
    with Assembly
    with Dependencies
    with DependencyGraph
    with GitVersion
    with NativePackager
    with Unidoc {

  override lazy val settings =
    super.settings ++
    gitVersionSettings ++
      Seq(
        organization := "com.rackspace.dcx",
        organizationName := "Autohost Team - GDCI - Rackspace Hosting, Inc.",
        startYear := Some(2013),
        scalaVersion := "2.10.2",
        resolvers += "spray repo" at "http://repo.spray.io/",
        scalacOptions ++= Seq(
          "-Xlint",
          "-Ywarn-all",
          //"-Xlog-implicits",
          //"-Xfatal-warnings",
          "-Yinline-warnings",
          "-deprecation",
          "-feature",
          "-language:existentials",
          "-language:higherKinds",
          "-language:implicitConversions",
          "-language:postfixOps",
          "-optimize"),
        testOptions in Test += Tests.Argument("-oD"))

  lazy val root =
    Project("foo", file("."))
      .aggregate(debian, unidoc, server, lib)

  lazy val debian =
    Project("foo-debian", file("foo-debian"))
      .settings(nativePackagerSettings(assembly in server): _*)

  lazy val unidoc =
    Project("foo-unidoc", file("foo-unidoc"))
      .settings(unidocSettings: _*)
      .aggregate(lib, server)

  lazy val server =
    Project("foo-server", file("foo-server"))
      .settings((assemblySettings ++ graphSettings): _*)
      .settings(
        libraryDependencies ++=
          Seq(
            akkaActor,
            akkaAgent,
            akkaSlf4j,
            logback,
            scalaTest,
            sprayCan,
            sprayClient,
            sprayIo,
            sprayJson,
            sprayRouting,
            sprayTestkit))
      .dependsOn(lib)

  lazy val lib =
    Project("foo-lib", file("foo-lib"))
      .settings(graphSettings: _*)
      .settings(
        libraryDependencies ++=
          Seq(
            argonaut,
            eaio,
            scalaCheck,
            scalaTest,
            scalazConcurrent,
            scalazCore,
            scalazEffect,
            scalazScalacheck))

}