Usually, when I read this passage from Romans, the message I hear is; "Practice what you preach!"  And I realize that, I'm only human - there are probably times when all of us, despite our best efforts to the contrary, stumble in exactly some of the areas we have preached on and know we should avoid.  Does this, then, disqualify us from preaching?

No. 

Instead, I think I can read this passage as saying; listen up preachers, because the Scripture text you preach on has a claim, not only on your listeners, but on you as well!  We preachers need to learn from the scriptures, we need to hear a word from God, whether that is a word of encouragement or admonishment.  The way you mess this up is to get into the pulpit prepared to preach on a passage, and to do so in a way that is aimed at others, without acknowledging that it must also be aimed at yourself.  There is a lesson in here (the Bible texts we preach on) for all of us!