Oh man, I feel like Dune Messiah (dune 2) is such a fantastic compliment to the original. It is about half the length and has such a different tone that makes the original even better.
Messiah is just good enough to make you think that the thrid book will be ok... it's is indeed ok. The third book is ok enough to make you think the fourth book will be ok. It's not. It's rotten.
My advice: Even though Dune 2 is a good book - it's best to stay off the slide downwards or you'll wind up like me reading all of them and wondering if I could get the time back.
I finished Dune about 2 months ago, and I'm currently a quarter through Dune Messiah. It so far is a great compliment and progression, and I constantly fight the desire to look up what happens because I want to know now.
Dune, followed by a 1-5 year pause, followed by Dune. Repeat until death.
You may, if you wish, throw in the first hundred pages of a different Dune novel every ten years or so, just to remind you of the correct sequence.