Here's how I do it.
I decided to use IPrincipal instead of IIdentity because it means I don't have to implement both IIdentity and IPrincipal.
- Create the interface - interface ICustomPrincipal : IPrincipal
{
    int Id { get; set; }
    string FirstName { get; set; }
    string LastName { get; set; }
}
 
- CustomPrincipal - public class CustomPrincipal : ICustomPrincipal
{
    public IIdentity Identity { get; private set; }
    public bool IsInRole(string role) { return false; }
    public CustomPrincipal(string email)
    {
        this.Identity = new GenericIdentity(email);
    }
    public int Id { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
}
 
- CustomPrincipalSerializeModel - for serializing custom information into userdata field in FormsAuthenticationTicket object. - public class CustomPrincipalSerializeModel
{
    public int Id { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
}
 
- LogIn method - setting up a cookie with custom information - if (Membership.ValidateUser(viewModel.Email, viewModel.Password))
{
    var user = userRepository.Users.Where(u => u.Email == viewModel.Email).First();
    CustomPrincipalSerializeModel serializeModel = new CustomPrincipalSerializeModel();
    serializeModel.Id = user.Id;
    serializeModel.FirstName = user.FirstName;
    serializeModel.LastName = user.LastName;
    JavaScriptSerializer serializer = new JavaScriptSerializer();
    string userData = serializer.Serialize(serializeModel);
    FormsAuthenticationTicket authTicket = new FormsAuthenticationTicket(
             1,
             viewModel.Email,
             DateTime.Now,
             DateTime.Now.AddMinutes(15),
             false,
             userData);
    string encTicket = FormsAuthentication.Encrypt(authTicket);
    HttpCookie faCookie = new HttpCookie(FormsAuthentication.FormsCookieName, encTicket);
    Response.Cookies.Add(faCookie);
    return RedirectToAction("Index", "Home");
}
 
- Global.asax.cs - Reading cookie and replacing HttpContext.User object, this is done by overriding PostAuthenticateRequest - protected void Application_PostAuthenticateRequest(Object sender, EventArgs e)
{
    HttpCookie authCookie = Request.Cookies[FormsAuthentication.FormsCookieName];
    if (authCookie != null)
    {
        FormsAuthenticationTicket authTicket = FormsAuthentication.Decrypt(authCookie.Value);
        JavaScriptSerializer serializer = new JavaScriptSerializer();
        CustomPrincipalSerializeModel serializeModel = serializer.Deserialize<CustomPrincipalSerializeModel>(authTicket.UserData);
        CustomPrincipal newUser = new CustomPrincipal(authTicket.Name);
        newUser.Id = serializeModel.Id;
        newUser.FirstName = serializeModel.FirstName;
        newUser.LastName = serializeModel.LastName;
        HttpContext.Current.User = newUser;
    }
}
 
- Access in Razor views - @((User as CustomPrincipal).Id)
@((User as CustomPrincipal).FirstName)
@((User as CustomPrincipal).LastName)
 
and in code:
    (User as CustomPrincipal).Id
    (User as CustomPrincipal).FirstName
    (User as CustomPrincipal).LastName
I think the code is self-explanatory. If it isn't, let me know.
Additionally to make the access even easier you can create a base controller and override the returned User object (HttpContext.User):
public class BaseController : Controller
{
    protected virtual new CustomPrincipal User
    {
        get { return HttpContext.User as CustomPrincipal; }
    }
}
and then, for each controller:
public class AccountController : BaseController
{
    // ...
}
which will allow you to access custom fields in code like this:
User.Id
User.FirstName
User.LastName
But this will not work inside views. For that you would need to create a custom WebViewPage implementation:
public abstract class BaseViewPage : WebViewPage
{
    public virtual new CustomPrincipal User
    {
        get { return base.User as CustomPrincipal; }
    }
}
public abstract class BaseViewPage<TModel> : WebViewPage<TModel>
{
    public virtual new CustomPrincipal User
    {
        get { return base.User as CustomPrincipal; }
    }
}
Make it a default page type in Views/web.config:
<pages pageBaseType="Your.Namespace.BaseViewPage">
  <namespaces>
    <add namespace="System.Web.Mvc" />
    <add namespace="System.Web.Mvc.Ajax" />
    <add namespace="System.Web.Mvc.Html" />
    <add namespace="System.Web.Routing" />
  </namespaces>
</pages>
and in views, you can access it like this:
@User.FirstName
@User.LastName