I’m trying to encourage my 63 year old brain to be better at French. I try and use idioms when I can, what’s your favourite everyday french idiom?
I find “desolé… mais je parle le francaise comme une vache espagnol” helps break the ice…
If this is a new one to you it means, i don’t speak french very well and references a view that basque people dont speak french very well…
Je m’en fous
C’est la fin des haricots !
crétin va
I like using odd phrases, many of which I make up myself by just translating apt Chinese idioms.
However, a true French one, that is less widely known but gets a good laugh down here. Now in our second year people know to expect and even repeat it to me all summer - Je transpire des carottes!
C’est dingue!
Complètement dingue!
Il pleut comme une vache qui pisse
Yes i like that one too!
Happily the xenophobia has been dulled down with basque turning into vache, but I wouldn’t use this phrase even in jest……
Mu current ear worm is
J’ai un coup de barre
Just “comme vache qui pisse” no une 
Someone expects you to give them something or do them a favour - Il peut se brosser= fat chance
J’en ai ras le bol = I’m fed up
Il a le cul bordé de nouilles = he is lucky
Il a un œil qui dit merde à l’autre = he has a divergent strabismus or by extension any lazy eye or squint
Thanks @vero - noted! The number of references to vache in Normandie is extraordinary but I suppose it’s not all that surprising…
Il/elle a son caractère.
That certainly gets bandied about… when not wishing to be drawn (about someone) and, depending on the tone of voice, it can be joking or serious… 
Rightly or wrongly… “mais, pas de neige” is what I offer when folk are moaning about the weather (or whatever) and that nearly always stops 'em in their tracks and respond with a smile/chuckle…
Ça m’en bouche un coin!
Ça t’en bouche un coin, hein ?
En voiture Simone ! (those with some older French TV culture will understand)
Prendre son pied…
Jeux de mains, jeux de vilains…
and the list goes on !
" Il faut se magner le popotin"
Which means, I think, “I have to get moving.” (Not strictly-speaking idiomatic)
Move yo ass 
Il faut faire avec…
“Les doigts dans le nez” - it’s easy to do.
“Renvoyer l’ascenseur” - return a favour.
“Je dis ça, je dis rien” - just saying
“le canard” is not only a feathered-friend, but a delicious cube of sugar soaked in eau de vie (hic) 
Also a ‘news’paper!
Yep… and I’d long thought that a padlock was a “canard” but that was 'cos folk here talk very fast and my brain simply chose the word which “cadena” sounded like…
"aha, it’s another duck "
Yet, it wasn’t until yesterday that a neighbour corrected me (with a smile of course)…
perhaps it was because he’s deaf and I was ennunciating my words (big mistake… 
)