How to execute an async task and recall itself after finished in the properly way

I need to create a GUI composed by textbox, checkbox etc where everytime I update any of them an async task calledSearchByFilter should start. This task must be called ONLY one by one, so for instance I update some UI controls there should be something like a "request pending" variable that should warn theSearchByFilter  task should restart once the previous call is completed.
To do this I will use reactive extension with .NET 4.0, so any suggestion compatible with these requirements would be even more appreciated.

Thank you