Sheets and Documents: Don't forget your driver's license, passport, insurance documents, a map and credit card/s. And if you leave Europe or the €EU, don't forget some foreign currency and a breakdown cover (just in case the unthinkable happens and you need roadside assistance).
Bike Gear and Tools: Don't leave without your tools, a chain lube, the tire pressure gauge and a good lock with a heavy chain. Also, don't forget to have your bike serviced before you leave for your adventure - it's important to make sure those tires and chain are up to the trip.
Rules and Regulations:
Make sure to get the stradal code information for the foreign countries you're visiting. For example, where in Europe is it mandatory to ride with the headlights on? You need to make sure you know, before you go.