A couple of years ago I mentioned the excellent Objective-C modelling framework Mantle that provides many nice features such as allowing you to quickly return a JSON dictionary representing an object.
Here’s a step-by-step example submitted by Atomic Object that will show you how to create a user model with Mantle to pass to a server.
Specifically the guide will show you how to set up the basic object, set up the required property keys by Mantle, set up a property transformer for data that cannot easily be serialized, and the actual serialization of the model.
You can find the guide over on the Atomic Object blog.
A nice straightforward guide for those getting started with mantle.