Here is my solution (Blog Example). I extracted this method out of the controller and use in in a "Helper" class:

    private static IEnumerable<BlogArticle> RelatedArticles(int id)
    {
        var umbracoHelper = new Umbraco.Web.UmbracoHelper(UmbracoContext.Current);
        IList<BlogArticle> list = new List<BlogArticle>();
        var service = ApplicationContext.Current.Services.RelationService;
        foreach (var relation in service.GetByParentId(id))
        {
            list.Add(new BlogArticle(umbracoHelper.Content(relation.ChildId)));
        }
        return list;
    }