I have done several editors... and the easier is using c# winform... it provides you with buttons, panels, listbox...
Of course, you can integrate it in a panel, like proposes @milkboat, but I think that winform series is not a good example.. because the xna code part is not driven as usual, there is no game class, and the code becomes ugly...
I prefer use this code, http://pastebin.com/kkkmKm8n, is not mine, but I've modified it to add the interface and the CreateAndShow method.
This way your game will not suffer many modifications among using the editor or not using it.
You only have to inherit your game class from XnaControlGame, and the editor form that will host the xna control should implement IXnaFormContainer.
I think is easy and more elegant than other approachs.... because let you difference xna part from the winform part
An editor with this approach:
An editor with a simple form approach, when the form is visible is editor mode, when dissapears is game mode.