There are a number of American expat groups here in France. Many of them seem to be based in Paris, but with members located throughout France (and they tend to distribute membership lists with members' addresses so you can find those closest to you).
Try some of the following, depending on your situation:
My wife and I are wanting to spend part of the year in France and part of the year back home in the US. We would like a furnished to partly furnished apartment in Lyon (at least 2br). How easy is it for an American to rent an apartment in France? Are there specific requirements or is it up to the landlord? Any recommendations for a real estate agent?
Residential rentals in France fall into two categories that basically split according to whether the flat is furnished or not. Furnished properties are governed by the rules for holiday rentals and hotels, whereas unfurnished have their own statute with lots of protections for the renter (i.e. to avoid the evil landlord tossing a family out into the snow in the dead of winter).
Furnished rentals can be limited to particular seasons, either because the property isn't equipped for the off season (e.g. not properly heated for winter occupation) or because the landlord charges a considerably higher rent (often weekly) for the peak season. Unfurnished rentals rent on a 3-year lease term (with specific terms and procedures for terminating the lease in the interim). It kind of depends how you are planning on scheduling your time in France and whether you want to rent something "full time" or if you're ok with completely moving out and moving back in with each visit to France.
But 3-year leases can be very difficult for newcomers to France. Most landlords expect to see three months of back pay slips (usually only French payslips) and evidence that your regular income is deposited into a local account each month. There is no central credit rating agency in France and your US credit history means nothing over here, so you sometimes have to get creative.
I'm actively involved with the American Club in Lyon, and it's a large organization with lots of very nice, welcoming, and helpful people in it. You can look at the calendar on their website to see all the activities they have every month, but it's enough to keep you busy. There are a great deal of Americans in Lyon now, so you would have plenty of company. As for renting, using a relocation company will really help you. We used MRI Relocation, and they are great. But like Bev said, that 6 months on and 6 months off thing is going to be tough to negotiate, and you'll most likely end up paying through the nose if you want a furnished place for just 6 months out of the year. You may do better just leasing a place year old. The good news about furniture is that french antiques are dirt cheap here, so you can fill your house up with beautiful pieces for less than you would pay at IKEA.
Is there anything going on in Lyon this Friday, Oct 11? We are coming up for the day to talk to a real estate agent and "check things out" without the kids to get a feel of city and where we'd like to live. We would love to meet fellow Americans, have a cup of coffee, etc...