Validation

I'm very happy so far with the Rails framework (at least compared to
J2EE). But I have some trouble
whith the valid?() method of ActiveRecord modell objects:
In the following code-snippet (behind a post-action the valid-method
sometimes is only the first-time
triggert. Pressing the back-button in the browser an submitting again
ignores the valid-action.

    @user = User.new(params[:user])
    @country = Country.new(params[:country])
    @address = Address.new(params[:address])

    @industry_overview =
IndustrieOverview.new(params[:industrie_overview])
    if request.post?
      # validation of the input field (before any save action)
      @user.valid?()
      @address.valid?()
   ...User.save(@user, @address) rescue puts "Save failed!"
Has anybody an idea?
Regards from Germany