Use before_filter

Bad Smell

class PostsController < ApplicationController
  def show
    @post = current_user.posts.find(params[:id])
  end

  def edit
    @post = current_user.posts.find(params[:id])
  end

  def update
    @post = current_user.posts.find(params[:id])
    @post.update_attributes(params[:post])
  end

  def destroy
    @post = current_user.posts.find(params[:id])
    @post.destroy
  end
end

In this example, the first code in action show, edit, update and destroy are the same, we hate the duplicated code, use before_filter to avoid.

Refactor

class PostsController < ApplicationController
  before_filter :find_post, :only => [:show, :edit, :update, :destroy]

  def update
    @post.update_attributes(params[:post])
  end

  def destroy
    @post.destroy
  end

  protected
    def find_post
      @post = current_user.posts.find(params[:id])
    end
end

As you see, all the post finders are removed from actions, and there is only one finder in the before_filter. Keep in mind that don't repeat yourself(DRY).

updated: in rails 4 or newer versions, you should use before_action instead of before_filter.