My wife gets a fancy French lifestyle magazine (I call it Franco-porn for visitors) and it had an article about how to shop at markets, and the author made a point that Rick also makes that I think bears repeating.
Always say Bonjour when entering an establishment or addressing someone. Always. And always say au revoir when leaving. It really is just how the French handle themselves, after standing in a pharmacy for an hour and seeing a coupla dozen locals come and go it is always done. And if you can toss in a bonne journee as you leave, which is the equivalent of Have a nice day, it seems to be appreciated as well. It doesn't mean you can't speak English after the intro, but start off with that.
And yes, you can do it in your best Julia Childs impression. It makes it fun.