I've followed Paquito's advice (see this post) on how to improve photorealism...
Here's a test scene, and I have to admit that it works quite well.
There's not enough details in this scene to really get a great image, but I'm sure that with a larger scene (and more practice) I'll be able to produce much better-looking images.
So thanks Paquito !
PS : GabrielFX also seems to be right about using Nik Viveza

1) Environment Map render pass 2) Daylight render pass 3) Post-production : the daylight pass was put on top of the EnvMap render, desaturated, I tweaked the levels, and mixed as "overlay" with an opacity of something like 30%