When it doesn't sound right no matter how you say it:

• Mark it and come back to it. Sometimes you overload your brain, you create an expectation that only finds fault. There often is no perfect way to read a sentence out loud, due to the choice of words. When two or more words are emotionally charged, one of them will be demoted slightly. If you come back to the selection later, you may find that a very good reading pops out. It's like looking at a Jumble word puzzle that you can't figure out. You come back and there it is, right in front of you.
• Read around it. Back up a couple of paragraphs and read to and through it right on to the next paragraph.
• Use boundaries formed by clauses and prepositions to make the sentence look like poetic verse. Use line breaks for pauses and take out the punctuation. The result may be a bit sing-songy, but it can help break the block you are having trying to find the right emphasis. Here's an example: “Instead of bending the stick, he grabbed it firmly and broke it into two pieces, then handed it back to her, smiling.”
  Instead of bending the stick
  he grabbed it firmly
  and broke it into two pieces
  then handed it back to her
  smiling.
• Pick any one word or word phrase and make it either louder or at a different pitch from the rest of the sentence.