The most common used strategy is known as "the red bet". With this strategt you stick to one colour (red or black). Due to the fact that the chance of winning each time is about 50%, you just keep doubling your bet each time you lose.
"The Lucky Number" is a strategy that only works if you stick to it with disciplin. You pick a lucky number before you start playing and stick to that number. You keep betting the same amount. Theoretically the ball hits your lucky number once every 37 (the amount of numbers on a roulette table) times. This means that if you bet 36 times the chance of winning is fairly high: 36/37=97,3%.
The memory game is not an official roulette strategy, but is still often used. On all the numbers (or on all the corners of the numbers) you put the same amount of chips; except for the last three numbers that hit. Because it is extremely rare that the ball hits the samen number multiple times in a row this strategy appears to work pretty well.