First stab at extracting breadcrumb trail building from the controller. Uses breadcrumbs_on_rails (https://github.com/weppos/breadcrumbs_on_rails) and attr_extras (https://github.com/barsoom/attr_extras).

base_breadcrumbs.rb

class BaseBreadcrumbs
  private

  def c
    controller
  end

  def edit?
    %w[edit update].include?(controller.action_name)
  end

  def new?
    %w[new create].include?(controller.action_name)
  end

  delegate :add_breadcrumb, :t,
    to: :controller
end

customer_breadcrumbs.rb

class CustomerBreadcrumbs < BaseBreadcrumbs
  pattr_initialize :controller, [:customer]

  def build
    add_breadcrumb t(:"admin.layout.breadcrumb_root"), c.admin_root_path
    add_breadcrumb t(:"admin.customers.index.title"), c.admin_customers_path

    if customer
      add_breadcrumb customer.to_s, c.admin_customer_path(customer)
    end

    if new?
      add_breadcrumb t(:"admin.customers.new.title"), nil
    elsif edit?
      add_breadcrumb t(:"admin.customers.edit.title"), nil
    end
  end
end

customers_controller.rb

class Admin::CustomersController < Admin::BaseController
  before_filter do
    CustomerBreadcrumbs.new(self, customer: customer).build
  end
  
  # …

  private

  def customer
    params[:id] && Customer.find(params[:id])
  end
end