You need to check that the country you're going to allows methdone to be bought into the country, some like Japan and Belgium do not. Always where possible travel with dry meds and keep them in your hand luggage so they can't get lost.
Take a letter with you from your prescriber stating what you're prescribed, what dose, and how much per day etc. In some countries they won;t let you fly with more than 100ml liquid even if it is medication, so take that into account.
As far as customs, I would offer up no information, as it's just another legally prescribed medication, like insulin or blood pressure meds. Only offer up the letter from the prescriber if they question you about the methadone, chances are they won't give it a second glance.
Indro is the definitive guide to travelling with methadone; http://www.indro-online.de/travel.htm