Outside bets in roulette carry the lowest risk. They cover more numbers yet the payouts are lower (1/1 for Red, Black, Odd, Even, 1 18, 19 36). These outside bets carry the best prospects for long-term wins.
The Martingale Strategy is arguably the best roulette strategy, and it's certainly one of the most famous ones. Simply put, you don't hold back when you lose; you do the opposite. Whenever you lose in roulette, the Martingale Strategy requires you to make your bet bigger than the previous one.