One of the best strategy of roulette is labouchere system
The labouchere system is a betting tactic like the martingale but does not require the gambler to risk his stake as quickly with dramatic double ups. The labouchere system involves using a series of numbers in a line to determine the bet amount, following a win or a loss.
The player adds the numbers at the front and end of the line to determine the size of the next bet.
When you wins, he crosses out the outside numbers and continues working on the smaller line. If you loses, then he adds his previous bet to the end of the line and continues to work on the longer line.