Since all game results are random and based on luck, no strategy can ensure you wager your bonus.
However, by understanding how casino game's work in terms of RTP (Return to Player) percentages and volatility, you may somewhat increase your chances of wagering your bonus by making your bankroll last longer.
To put it simply, the best strategy is to play low-volatility slots with high RTPs and adjust your bet size according to your current bankroll.