Good architecture for desktop client application

I've already fallen several times into issue of creating desktop client app for working with some server and every time I ended with ugly code, which just impossible to support after couple of releases.

I have highlighted the following key points: - All operations must be asynchronous, without any dummy windows for relative fast operations (<30sec) - App has to connect with server from time to time and check, for example, user account - All heavy operations must be cancelable But, most important, all of this must be "naturally" in code, without creating unnecessary difficulties (singeltones, hacks, etc).... only really needed code with minimal overhead

How do you design such kind of app? What pattern you use? What opensource project with good architecture you can recommend?

P. S. Sorry for my bad English.