Can't speak for Corsica, but Zakynthos nannys were great. We had a creche meet and greet the afternoon we arrived, where the kids all played together and got to meet the nannys while the head nanny showed us around the creche and explained everything. Also, before going we had been asked to fill in a form detailing ds' routine, likes and dislikes etc etc - this had been sent ahead of us and the girls seemed really well briefed.
Anyway, on the first morning ds was bit clingy, so just stuck around with him for a while until he relaxed and started playing with the other kids and staff. Then he told me to go! We went through this little routine each morning - I'd come in and play for five minutes and then he'd say I could go. When I signed him in each morning I had to leave a contact number if we were planning to go off site - so I knew that they would either come and get us or call us if there were any problems. But fortunately they never needed to as he was very happy.
Think it helped immensely that the girls were so lovely - they really warmed to ds and he to them. (By the end of the holiday ds was doing impressions of the head nanny - much to the other girls' amusement!) They really worked hard at making it as much fun as possible for the kids - loads of games, water and sand play, sticking, painting, foot painting, songs, trips to feed the chickens etc etc.
Think it also helped that ds had been at nursery for about six months by the time we went away - so he was pretty good about being left. Also it meant I knew that any initial upset would be forgotten by the time I got to the door! I think sykes is right about having to be tough. I honestly don't think it helps if you hang around too long - because children aren't stupid! They know you're going to go at some point, so better for your goodbyes to be relatively short and sweet than some drawn out affair that still has the same ending!
So top tips would be make sure the staff know a bit about your child's likes and dislikes, stick around for a short while until they seem settled and then peg it. Remember, you can always pop back for a sneaky peek to check they're ok - and someone will come and get you if they aren't!