From http://blogs.msdn.com/b/powershell/archive/2008/09/02/speeding-up-powershell-startup-updating-update-gac-ps1.aspx

ngen.ps1

Set-Alias ngen (Join-Path ([System.Runtime.InteropServices.RuntimeEnvironment]::GetRuntimeDirectory()) ngen.exe)
[AppDomain]::CurrentDomain.GetAssemblies() |
    ? GlobalAssemblyCache -eq 0 |
    ? Location |
    % { ngen install $_.Location }