I have a restful URL for the action of editing a page. This is implemented on the controller as an Edit method, which accepts GET requests and an Edit method that accepts PO
Even though Ken's answer does highlight an important fact - PRG doesn't necessarily mean "blindly return a redirect when posting" - you still might want to do redirect and preserve modelstate sometimes.
The easiest way to handle that scenario, is using action filters to export modelstate to the session (before redirecting), and then import modelstate (before executing the new action). Kazi Manzur Rashid has a couple of excellent blog posts (Part 1 Part 2) on best practices in ASP.NET MVC. They're quite old, but many of the tips there are still very much applicable. Tip number 13 in the first article is exactly what you're looking for.