public partial class Startup
{
    // For more information on configuring authentication, please visit http://go.microsoft.com/fwlink/?LinkId=301864
    public void ConfigureAuth(IAppBuilder app)
    {
        // Configure the db context and user manager to use a single instance per request
        app.CreatePerOwinContext(ApplicationDbContext.Create);
        app.CreatePerOwinContext<ApplicationUserManager>(ApplicationUserManager.Create);

        // Other things...
    }
 }

public class AccountController : Controller
{
    private ApplicationUserManager _userManager;

    public AccountController() { }

    public AccountController(ApplicationUserManager userManager)
    {
        UserManager = userManager;
    }

    public ApplicationUserManager UserManager {
        get
        {
            // HttpContext.GetOwinContext().Get<ApplicationDbContext>(); // The ApplicationDbContextis retrieved like so
            return _userManager ?? HttpContext.GetOwinContext().GetUserManager<ApplicationUserManager>();
        }
        private set
        {
            _userManager = value;
        }
    }

    // Other things...
}