I'll second the chlorine bleach recommendation.

1 tablespoon bleach per gallon
3/4 teaspoon bleach per quart (1000 ml)
1/4 + 1/8 teaspoon bleach per pint (500 ml)

First clean the bottles with hot soapy water. Rinse well.

Add warm (not hot) water, add chlorine bleach, cover, and shake. Turn the bottle over and unscrew the lid just enough to let some chlorine solution cover the outside thread area of the bottle. Screw the lid back on and then allow solution to sit for 30 minutes - though most of the sanitation will happen in the first 2 minutes. Poor the solution into the next bottle and repeat. Rinse the bottles with clean tap water and then air dry on a clean surface.