In ASP.NET MVC, I typically use View Models along with the Json.NET library to get JSON back to the client.  When I want some members to be omitted from serialization I will simply add the [JsonIgnore] attribute to the target property.  This works great for static omission but sometimes I come across the need to perform conditional (i.e. at runtime) omission.  To do this, you can use the largely undocumented [bool *ShouldSerialize{MemberName}()] *approach.

Simply add a bool return method named ShouldSerialize{MemberName} to the model class.  The {MemberName} portion of the name should match the member name that will be conditionally omitted.  Json.NET will check for the existence of this method and conditionally serialize the corresponding member value.  I also like to use an additional property (with [JsonIgnore]) that I set externally which is used by the ShouldSerialize methods.  In the example below, this property is called SerializeSensitiveInfo.

public class UserViewModel {

  public string FirstName {get;set;}
  public string LastName {get;set;}
  public string Username {get;set;} //conditional serialization!
  public string SecurityQuestion {get;set;} //conditional serialization!

  public bool ShouldSerializeUsername(){
    //only serialize Username if SerializeSensitiveInfo == true
    return (this.SerializeSensitiveInfo);

  public bool ShouldSerializeSecurityQuestion(){
    //only serialize SecurityQuestion if SerializeSensitiveInfo == true
    return (this.SerializeSensitiveInfo);

  public bool SerializeSensitiveInfo {get;set;} //this property allows control of serialization at runtime