This is not intended to be an exhaustive list. Just a starting point to avoid headaches while you are traveling. Please send updates, comments, suggestions!
1. Photocopy passports/credit cards/prescription medicines.
Leave one copy here and bring one with you.
2. Have enough medicines with you for your entire trip. Don't check them with your bags. Duh.
2. Make sure you have at least 6 mos on your passport (before it expires) from your return date. THIS IS IMPORTANT. While some European countries are lax and don't inspect your documents upon entry, some are particular. My mom almost (it was very nervy and the officer starting ranting about President Bush, but decided to let us board anyway) didn't get to board the Eurostar (Paris bound for London) because she didn't have that 6 mos cushion.
3. Just in case, know where your embassy is.
US Embassy in Paris:
4 avenue Gabriel
75008, Paris
metro: Concorde or Invalides
4. FYI: the last 2 digits in Paris zip codes will tell you what arrondissement the location is. Hence, the US embassy is in the 8th arrondissement.
5. Money one. Take enough euros to get by for a day. Then use ATM's. You will get the best exchange rate and lower your per transaction fees. GUARD your pin. Even better, GUARD your pin and use ATM's in less touristy areas.
6. Money two: Call your credit card companies before you go. Tell them you will be out of the country, where, dates......or they may cut you off until you can pass their security line nightmare. Some may ask about how much you plan to spend. ASK about foreign currency conversion FEES, and per transaction FEES. I use my debit card for substantial purchases (sigh, no air miles) and euros from ATM's for smaller purchases.