Agree on debit/credit cards, cheaper than Travellers’ Checks and easier. About a year ago, a travelling companion had travellers checks (we were in Oregon!) and the clerk didn’t know how to deal with it. And definitely call the bank(s) that issues the card(s) to tell them where/when you’ll be – otherwise, they’re liable to spot it as a risky transaction and declare your card unusable.
Write up a list of all the credit cards and the phone numbers where to call if they’re lost or stolen. Keep that list somewhere safe and separate from your wallet and cards.
Take ONE card (I’m assuming you have several) and hide it somewhere in your luggage. That way, if you get pick-pocketed or otherwise lose your wallet/purse, you’ll still have one good card. (My wife and I travel together, and we each have one card that the other one doesn’t have, for the same reason.)
Make a couple of photocopies of your passport, and hide them somewhere in your luggage. Your passport is by far the most valuable thing you are carrying; and if it gets lost or stolen, you’re up Shit Creek sans paddle… unless you have the passport number. It will still be a major headache if your passport is lost/stolen, but if you’ve got a copy of that first page, it will be way less of a headache.
Don’t carry anything with you that you can’t afford to lose. France isn’t a “problem” area in terms of pick-pockets and thieves, but, frankly, any place has such risks. It’s more of a headache to be robbed in a foreign country, where you’re not familiar with the laws, languge, procedures, etc. So, take sensible precautions: don’t wear 'spensive jewelry, etc. It’s the same precautions I take when traveling in the US, as well.