Official documents:

Passports:
Check your passport and make sure it's valid for at least 6 months after the return from your trip! Most countries today will not let you in if your passport’s expiration date is less than that.

Visas:
Make sure that you check the entry requirements for all people in your party. Remember, different nationalities have different requirements. A Green Card will allow you to return to the US but will not provide the same rights as an American Passport.
VisaCentral

Print and bring your voucher!
Your printed vouchers are proof of payment. Remember, each service may require a different type of voucher whether it be electronic or paper.

Reconfirm your flights:
We recommend that you reconfirm your flights with the airlines a minimum of 48 hours prior to your departure and return dates. This will ensure that you are aware of any airline schedule changes that would possibly compromise your travel plans.