There is one technique that I rely on more than anything else when teaching swimmers the mechanics of swimming. It's sculling. Most sessions will have at least a few lengths of sculling, whether it's part of a warm-up, cool down or even part of the main set. Learning how your body moves, manipulates, and feels in the water is a crucial part of increasing both your speed and efficiency in the water.
I've always felt that Breaststroke was seen as the lazy cousin to the other three IM strokes within the swim community. Some believe it to be the easiest stroke and the slowest. Unfortunately, I can't argue with you on Breaststroke being the slowest. Just the natural movements of Breaststroke make it slower and less streamlined than its counterparts, but this doesn't mean that it's for the lazy or is easy to do.
One issue I've faced over the years is knee pain among swimmers who swim breaststroke. Knee pain in swimmers, even though our sport is very low impact, is very real. This should never be ignored by an athlete or a coach. It has been documented that 73% of Breaststrokers and 48% of non-breaststrokers all suffer from some sort of knee pain. That's a lot of swimmers.