How can I modify LabelFor to display an asterisk on required fields?

I did that way because my required fields must be dynamic (defined in a configuration file)

Add at the end of your View:

    <script type="text/javascript">
        $('input[type=text]').each(function () {
            var req = $(this).attr('data-val-required');
            if (undefined != req) {
                var label = $('label[for="' + $(this).attr('id') + '"]');
                var text = label.text();
                if (text.length > 0) {
                    label.append('<span style="color:red"> *</span>');
                }
            }
        });
    </script>

Here is my solution based on Adam Tuliper's answer but modified to work with Bootstrap and also allow the usage of custom attributes.

using System;
using System.Linq;
using System.Web.Mvc;
using System.Linq.Expressions;
using System.ComponentModel;


public static class RequiredLabel
{
    public static MvcHtmlString RequiredLabelFor<TModel, TValue>(this HtmlHelper<TModel> helper, Expression<Func<TModel, TValue>> expression, object htmlAttributes)
    {
        var metaData = ModelMetadata.FromLambdaExpression(expression, helper.ViewData);

        string htmlFieldName = ExpressionHelper.GetExpressionText(expression);
        string labelText = metaData.DisplayName ?? metaData.PropertyName ?? htmlFieldName.Split('.').Last();

        if (metaData.IsRequired)
            labelText += "<span class=\"required\">*</span>";

        if (String.IsNullOrEmpty(labelText))
            return MvcHtmlString.Empty;

        var label = new TagBuilder("label");
        label.Attributes.Add("for", helper.ViewContext.ViewData.TemplateInfo.GetFullHtmlFieldId(htmlFieldName));

        foreach (PropertyDescriptor prop in TypeDescriptor.GetProperties(htmlAttributes))
        {
            label.MergeAttribute(prop.Name.Replace('_', '-'), prop.GetValue(htmlAttributes).ToString(), true);
        }

        label.InnerHtml = labelText;
        return MvcHtmlString.Create(label.ToString());
    }

}

Then, I call it from my view like this:

@Html.RequiredLabelFor(model => model.Category, new { @class = "control-label col-md-3" })

P.S. Make sure you don't forget to include your namespace in your view.

Here is an blog post that describes how to do this.

To give you a small example modified from the site above (note - I have not compiled/tested this):

namespace HelpRequest.Controllers.Helpers
{
   public static class LabelExtensions
    {
        public static MvcHtmlString Label(this HtmlHelper html, string expression, string id = "", bool generatedId = false)
        {
            return LabelHelper(html, ModelMetadata.FromStringExpression(expression, html.ViewData), expression, id, generatedId);
        }

        [SuppressMessage("Microsoft.Design", "CA1006:DoNotNestGenericTypesInMemberSignatures", Justification = "This is an appropriate nesting of generic types")]
        public static MvcHtmlString LabelFor<TModel, TValue>(this HtmlHelper<TModel> html, Expression<Func<TModel, TValue>> expression, string id = "", bool generatedId = false)
        {
            return LabelHelper(html, ModelMetadata.FromLambdaExpression(expression, html.ViewData), ExpressionHelper.GetExpressionText(expression), id, generatedId);
        }

        internal static MvcHtmlString LabelHelper(HtmlHelper html, ModelMetadata metadata, string htmlFieldName, string id, bool generatedId)
        {
            string labelText = metadata.DisplayName ?? metadata.PropertyName ?? htmlFieldName.Split('.').Last();
            if (String.IsNullOrEmpty(labelText))
            {
                return MvcHtmlString.Empty;
            }
            var sb = new StringBuilder();
            sb.Append(labelText);
            if (metadata.IsRequired)
                sb.Append("*");

            var tag = new TagBuilder("label");
            if (!string.IsNullOrWhiteSpace(id))
            {
                tag.Attributes.Add("id", id);
            }
            else if (generatedId)
            {
                tag.Attributes.Add("id", html.ViewContext.ViewData.TemplateInfo.GetFullHtmlFieldId(htmlFieldName) + "_Label");
            }

            tag.Attributes.Add("for", html.ViewContext.ViewData.TemplateInfo.GetFullHtmlFieldId(htmlFieldName));
            tag.SetInnerText(sb.ToString());

            return MvcHtmlString.Create(tag.ToString(TagRenderMode.Normal));
        }
    }
}

You can add an asterisk to a required field purely through CSS.

First, create a CSS class for it:

.required::after 
{
    content: "*";
    font-weight: bold;
    color: red; 
}

This will append a red asterisk to any element with the "required" class.

Then, in your view, simply add the new class to your label:

    @Html.LabelFor(m => m.Name, new { @class="required" })

Even better might be a custom HTML Helper that discerns if the field has a [Required] attribute, and if so, adds the required CSS class.