How to win at roulette? I know that the answer to my attention-grabbing question is : "You can't win at roulette, it's a negative Expected Value game".
Yes, you're right, long term speaking.
Let's imagine a medium-short term situation (just an after-dinner at the casinò) where there will be <100 bets.
What's the best strategy to use to win some money?
I imagine a hypothetical formula that considers our "money target" and percentage of success as inversely proportional (the more we want to win the less likely we are going to succeed).
What do you think? Are those "red-black double if lose" systems useful?

The negative expected value applies to short term as well. You simply do not have the odds.
Read up on Martingale, it is the system you mention at the end. Even that is not going to help you. If you lose too many times in a row and do not have enough cash, you lose big time.
So it is true, it is a negative EV game. Play for fun, not for sure profits. 
Edit: If I had to, I would play a game where expected win is around zero and does not require great capital. That is e.g. betting a fixed amount on the same color over and over (or switch colors, does not matter). You should be about even (zero disrupts it), but if you get lucky, you might win/lose something. 
