Don't do anything.

I made the mistake once to bring an old car to a garage to have it prepared for the "contrôle technique". They replaced the (perfectly functioning) headlights with the excuse that the alignment system was broken. The garage did not ask me if it was ok to replace the piece, although I explicitely asked them so, and presented me with a bill of 600 CHF. Almost surely the car would have passed the inspection without this fix.

Fast forward 2 years, and I bring the car to a different garage for another "contrôle technique". This time they tell me that the car needs 4500 CHF in repairs, which was three times the value of the car. So I go to the SCV without doing any fix, hoping that the car wasn't in such a bad shape that the authorities would immobilize it there... and the car passes the test with flying colors.

Moral of the story: have your car serviced regularly, keep it in good shape, but don't do anything special for the "contrôle technique". The SCV directly will tell you which exact fixes, if any, your car needs; then you go to the garage, have these repairs done, and take the SCV test again (and note that most fixes do not even need you to take the test a second time).  

The objective of the Control Technique is to verify that there is nothing wrong with your car, so as to avoid it breaking down on the road and creating a risk for other drivers (not to mention for you) or to stop you from operating it in a way that it creates a risk (such as if your lights are malfunctioning) or excess emissions. All relevant systems are checked. Anything that needs repair is noted and if something is found, you have to return after repairs, so you can pass. There is NO penalty for not passing the CT, you just have to make the necessary repairs.

If on the other hand, as OmegaMan pointed out, you go to a mechanic in advance, they can smell a s****r and will scare you into accepting unnecessary repairs. Some will go ahead and make "repairs" without your prior consent if you sign an authorization in advance to bring the car to compliance or someting like that. 

Didn't you ever do an MOT test in the UK? Or did you always drive cars less than three years old?

I agree with the above and had mine yesterday - was in and out in under 15min. My car is in reasonably good shape to be fair and like you, I had it serviced recently with no issues raised. I also heard that it speeds up the process if you pay the fee/tax well in advance so they have it processed and logged on the system when you come (rather than a day or two before and going there with proof of payment).

One last small point. There are roadworks around there at the moment so give yourself 5-10min extra to arrive on time depending on which direction you're coming from.

That's wonderful, thank you very much for all your helpful comments!

Just to confirm what I wrote:

My car passed the periodic control a few days ago.

I didn't do anything to prepare it, apart from washing the bodywork, which you can do by yourself in any "lavage auto" for around 4 CHF. Control done in 15 minutes, everything was ok.

A side note: the contrôle technique is not done anymore at the OCAN but at a certified private garage which you can choose from a small list (everything is explained in the letter you get from the OCAN).