First tip...don't exchange cars...rent your own car. Many home exchangers exchange cars, as well as homes. In theory, this is a great idea and another way to save money. Here are a few reasons it's a better idea in theory than in practice.
1. If you accidentally put the wrong type of gas in the exchanged car...you could end up having to replace an engine. With a rental car...you don't have to worry about it.
2. You don't really know what shape the car you are borrowing will be in. It could be a brand new car with no issues...or it could be an older car with specific requirements (ie...you can only put in a little oil, don't fill it up or it won't run properly) that you don't find out until you've already done what you weren't suppose to do.
3. Things can get broken on your car because no one takes care of your car like you do. For example, a small tear in a convertible top becomes a very large tear because you weren't there to show your exchange family how to carefully put the top up and down.
4. Speeding Tickets...yep...you or the exchange family can get radar tickets and not even know it until you're back home (this, luckily didn't happen to us, but it was a constant worry for our host, as he had the maximum points allowed on his license and if his car got anymore tickets...he'd lose his license. Needless to say, we tried not to speed...which really wasn't a problem since we had filled up the oil on the car because it was low, the day before we found out that the car was temperamental and you should only fill it up half way or it would be sluggish (translated...no way you could do the speed limit on the freeway without a big downhill running start!) Or, someone could get a ticket in your car for making an illegal u-turn on the freeway (this did happen...in the US...not France...we're waiting to see if this becomes a problem for us.)
5. Parking Tickets...no need to really say anything here. It's fun to come home and see what surprises there are in the mail! (For full disclosure, our exchange family did pay for all the parking tickets they got.)
6. Your tags could get stolen and your exchange family gets a ticket for expired tags. Yep, someone stole our tags on the Expedition and the exchange family got a ticket. I felt bad thinking we had forgotten to renew the tags, but turns out they were stolen off the truck.
With a rental car, none of these things are issues. If you get a ticket, you deal with it. There's no risk that your exchange family will have any problems from your tickets. You almost always get a fairly new car, so it doesn't break down...and if it does...you can call someone who speaks your language and have it dealt with.
In conclusion, it's probably not a terrible idea to exchange cars for a one or two week exchange (but all of the above are still risks)...however for anything longer, I recommend renting your own car! Who knows, you might even get to rent something like this: