How do you avoid "boring" once you're an established author?

This is not the first time I have read a popular author's sequel book and found it, well....boring.

For the record, and for the salvation of my future as a published author, I will not divulge as to what book I am referring too. And no, it is not the book I have listed to the right in my "Currently Reading" sidebar.

What I wonder is, how does this happen?  How does one go from great to eh, okay.  Could it be that their popularity and fame has deluded editors judgement?  Or could it be that I read the author's other books a long while back but now find the writing subpar?  Maybe but beyond my opinion, others, a majority others praise this author.

So I ask, how can we avoid this as future authors?  I'd like to think we would not get so full of ourselves.  That we won't let book sales go to our head and we will still strive to write a better book than last time.  That beating a dead horse is simply that, beating a dead horse.  I hope than when I'm published and WHEN I get my three book deal (You like that putting-it-out-in-the-universe mentality?) that I preform like Susan Collins and write the greatest Mockingjay of my life.  That's how you do it!

How will you crack the "Sequels Suck" myth?