I would like to quote myself from http://www.devexpress.com/issue=Q488085 as I believe that many other XAFers will be interested in this discussion:

"There are several good ways to store settings in XAF:
1. Use a persistent singleton class

2. Use the Application Model
For instance, you can implement a custom model extension that will hold the value and then create another element that will have a list of the first elements. Of course, the application model can be physically stored in the database, if required.

3. Use the ValueManager class (How to save an application's settings at runtime and then access them via a common interface from both Windows Forms and ASP.NET platform-dependent modules (Example)).

4. Use any other solution you would use for the same task in a regular non-XAF application."

Personally, I think that #1 and #2 are good.

Feel free to leave comments if I missed some cool places or ways to store settings;-)