I don't think I can offer many specific phrases.. I'm just back from nearly three weeks in France and agree with Yello and Asterix - have a try, be self effacing about your lack of language and smile a lot, and they will be pleased you tried and make the effort to be understood back. My friend (who has a house there) did pass on the word "douchement" to me - 'gently' - to be used in the context of someone speaking french back to you too fast for you to understand.
A tip - almost any word ending in 'ion' will mean the same as in English - there are about 3 exceptions. Exception is not one of the exceptions.
To feel like you're really blending in, when you say au revoir, say it sort of slurred in to one - "auvoir". I found that once I'd got a few familiar words pronounced in a french way, rather than in a stilted English way, I felt more confident. I had schoolbook French from many years ago, and some experience from a trip two years ago, and felt much more confident after a week immersed in the language.
oh, 'pas probleme' (pas pronounced without the s, as 'pa') - no problem, no worries. I used this at the end of an unsuccessful attempt to get directions from an old chap leaning on a gate - although I was still lost (he understood me, I half understood him, but he just didn't have the answer to my query). It just felt better to say "ah, well, no worries, thanks" than to just drive away...
Also, on a cultural note, be prepared if you're in a cafe or bar/tabac, especially in little villages, for people to walk in and greet the whole bar and recieve 'bonjour' from everyone in there. Join in with a bonjour - there is a wonderfully old fashioned formality-yet-friendliness about rural France. And most bars don't sell pastries or such like, but are often happy for you to eat one you've brought with you, while you drink your coffee. I never did the asking but I think it went something like "Si possible que manger notre croissants, s'il vous plait?" For you on your own it would be 'ma croissant', I think (phonetically - Si posseeb, k' manjer ma croissant, si vou play?) Waving your bag of buns and grinning should help. Tip is to get the buns when you first get on the road - boulangeries often sell out early, and you want them nice and fresh...
