new.html.haml

- form_for @user do |f|
  = f.hidden_field :signup_step, :value => @user.signup_step
  = render :partial => "step_#{@user.signup_step}", :locals => { :f => f }
  = f.submit

user.rb

class User < ActiveRecord::Base
  attr_accessor :signup_step
  validates_presence_of :first_name, :last_name, :billing_address
end

users_controller.rb

class RegistrationsController < ApplicationController
  STEP_VALIDATIONS = [
    %w(first_name last_name),
    %w(billing_address)
  ]
  
  def new
    @user = User.new
  end
  
  def create
    @user = user_from_session
    
    increment_signup_step @user, params[:reg]
    
    respond_to do |format|
      if @user.signup_step == STEP_VALIDATIONS.length && @user.save
        format.any { render :final, :status => 202 }
      else
        format.html { render :new }
        format.any(:js, :json) { render :json => @user.errors, :status => @user.errors.empty? ? 202 : 400 }
      end
      
    end
  end
  
  private
  def increment_signup_step(model, params)
    model.attributes = params
    
    model.signup_step += 1 if valid_for_attributes? model, *STEP_VALIDATIONS[model.signup_step]
  end
  
  def valid_for_attributes?(model, *attrs)
    return true if model.valid?
    
    errors = model.errors.dup
    model.errors.clear
    
    errors.each do |attr, message|
      model.errors.add(attr, message) if attrs.include? attr
    end

    return model.errors.empty?
  end
  
  def user_from_session
    session[:temp_user] ||= User.new
  end
end