The first is to eliminate resistance caused by the water to your forward momentum. You do this by improving body balance, position, and alignment. Clean up your body position, and even if you don't make any changes in your stroke, your still-imperfect stroke will move you further and faster. The second way is to create more propulsion by locating your power source and using your hands more effectively. So get to work first on the skills that eliminate resistance, because they offer more opportunity for improvement.
Longer boats go faster than shorter boats.
The same goes for your body. The longer your body during all phases of all strokes, the better it will hold its speed during the slight pauses between strokes. That keeps your body longer while your other hand completes its stroke and takes over the front position. In breaststroke, after pulling, stretch and streamline both arms forward and keep them there until after you complete your kick. Longer body equals faster body.