You can try my ServiceStack JsonSerializer it's the fastest .NET JSON serializer at the moment. It supports serializing DataContract's, Any POCO Type, Interfaces, Late-bound objects including anonymous types, etc.
Basic Example
var customer = new Customer { Name="Joe Bloggs", Age=31 };
var json = customer.ToJson();
var fromJson = json.FromJson<Customer>();
Note: Only use Microsofts JavaScriptSerializer if performance is not important to you as I've had to leave it out of my benchmarks since its up to 40x-100x slower than the other JSON serializers.