We didnot play with our own money, but with the coupons on the back of the cinema tickets. But even so, we should have made slightly less than 300 a month, but we did a slightly more on average.
We did not really have an optimal betting strategy, at least nothing that can be proven scientifically, there is no such thing. But after some time we started to bet on the wheel, e.g. "Voisins du zero" or "Jeu zero" and we knew all the croupiers and some of them had an unbelievable regular way of running the ball and spinning the wheel and the ball would thus drop in a slightly more predictable area of the wheel - at least when it hit one of the bumpers and would then drop into the wheel. So we waited with our bet until our favourite croupier would throw the ball from a place, where we felt it should land close to the zero and call a zero game.
Naturally that did not always work and we'd have losing streaks of even two weeks where we went home without any money at all, but once a month or so we'd go out with over a hundredfifty euros. On average we did slightly better than expected (400 were admittedly the better months, sometimes it was a little less than three hundred), but we remained ahead even after a long time, so my guess is we were either lucky or the idea of predicting according to the croupiers regularity worked. We had the feeling it worked on two of the ten croupiers the others threw less regular.
Anyway it was definitely more fun than just whitewashing the chips. (One was not allowed to immediately exchange the "lucky" chips one would get for the coupons for money.)