While you can get pretty scientific about your training by incorporating lactate threshold numbers or by measuring the watts you generate during training with a power meter, there are a few simple tried-and-true training methods that have been used by cyclists for years to develop a more powerful pedal stroke.
By using larger gears for short amounts of time, you can build the strength in your legs and core that's necessary to get faster. These intervals won't feel terrific while you're doing them, but after two to three months, you should start to notice a difference in your power and speed on the bike.
*Please note that if you've had an injury to your knee, use caution when training with larger gear ratios, as it can worsen symptoms.