Step 1: Aquiring a Sealed Cap
Gently remove the cap(squeeze the sides and twist) from the new bottle without breaking the perforated seal. Replace that cap with your old one.
Step 2: Wash and Fill Empty Bottle
Over fill = good.
Under fill = suspicious.
Keep that in mind ;)
Step 3: Re-Cap
Pack in your suitcase just like you normally would your mouthwash.
For those of you attempting with a container that's not opaque (this mouthwash contains hydrogen peroxide - which is photosensitive), add a few drops of food coloring ;) It won't change the flavor, but will become a very potent stain adder. So be careful ;)
Step 4: Implications
Similarly, this could be used for other forms of "evil." I hope you're not too paranoid to think that the cutting edge of this evil is college spring break at sea ;)