This question was asked by David Semaria on Mark Suster's excellent blog "Both sides of the table", a must read for anyone who needs to pitch to VCs.
Here is my take on the question why TED presentations are so "polished":
  1. A tough pre-selection: you need an interesting story even before the PPT slideware is opened to create the presenation
  2. A ruthless 18 minutes cutoff makes you practice
  3. Peer pressure of a good speaker line up makes you practice
  4. The "threat" of a global video audience makes you practice
You can argue that it can be hard to sometimes to meet point number 1. Number 2, 3 and 4 are all about practice, your presentation can benefit from it too. There is no excuse not to practice, practice, and practice.