Two words: PACK LIGHT. Of course, what you pack will depend on a lot of things—the length of your trip, where you're going, how strong your arms are. But no matter what, you want to pack light. Bring what you think you need, then cut it in half. Remember, you'll be carrying your luggage from the bus to the hotel (many times on longer trips), and not all hotels have elevators. Also, don't bring any suitcase bigger than 29". Below is a generic packing list to help you get started. This is just a guide and isn't tour-specific.
1.Clothes (socks, underwear, t-shirts, pants, sweaters if you're going somewhere cold)
Bring a laundry bag to separate the dirty clothes from the clean
clothes in your suitcase. Rolling your clothes (as opposed to folding 
them) saves space and helps you avoid wrinkles.
2.Passport
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3.Photocopy of your passport
Leave a photocopy of your passport with someone at home too.
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4.Copies of your flight and hotel itineraries
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5.Comfortable walking shoes (sneakers/tennis shoes are good)
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6.One or two dressier outfits. Some clubs have strict dress codes (no jeans/no sneakers). It's a good idea to bring a nice pair of pants and shoes.
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7.If you're traveling in summer, bring a sweater or a light jacket, as it can be cool at night.
If you’re going inside the Vatican, you have to have your shoulders covered. A shawl for girls will do the trick. Also, skirts and shorts must fall at or below knee level.
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8.iPod
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9.Camera, batteries
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10.Converter and adapter – You’ll need these to use any electrical items (for example a hair dryer or iPod charger). You can find these at any travel store.
Check the voltage on your hair dryers, curling irons and straighteners. No one likes to see their Chi light on fire in Rome.
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11.Cell phone – If you plan on bringing your cell, ask your provider about international service and the fees associated with international calls.
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12.Credit/debit cards – We recommend using credit/debit cards and ATMs as your go-to money source.
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13.Cash – Before your trip, go to your bank and exchange some USD ($100-150) to the local currency of the country you’ll visit first.
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14.Medication – Bring enough for the length of the tour plus two or three days. Pack some in your carry-on bag in case you get separated from your luggage. You should also bring a copy of your written prescriptions.
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15.Sunscreen
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16.Sunglasses
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17.Umbrella
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18.Travel journal
Instead of a travel journal I buy a postcard in each country and jot down what happened on the back. Takes up less space.
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19.Addresses of friends and family for postcards
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20.Washcloth/towel (optional) - Many hotels in Europe do not provide washcloths in the rooms. Bath towels and hand towels are always available, but don’t count on a washcloth. Bath towels may be smaller/thinner than you’re used to.