Mega 2560 problem again. Please help!
Do you have another Mega board to test with? That way we can tell if it's just this specific board. Is it timing out? Something like...

avrdude: stk500_2_ReceiveMessage(): timeout


It could very well be a driver issue. Give this a go:


  Mac:
  
  1) Go to System Preferences -> Network
  
  2) If it asks you to add the Arduino (modem device), say cancel
  
  3) Remove the Arduino device from the list if its there
  
  4) Try uploading a sketch!
  
  PC:
  
  1) Go to Device Manager
  
  2) Find your Arduino in the list, go to its properties and update its driver manually, using the .inf file which came with your Arduino (0021+) IDE


Also, make sure you're not sending !!! via serial at any point in your sketch as that initiates a special monitoring mode baked into the bootloader.

Source

Also, take a look at the official Arduino troubleshooting guide.