Sparring is the best way to find out where your weak point is. If you are consistantly hit in the same area, you know that this is where you must develop a better defense stategy. Your basic blocks are improved simply through practice, practice, and more practice. Remember that offense is your best defense. I always tell people that there are 3 stages to a martial artist's defense. 1. learn to block, then counter. 2. learn to block and counter at the same time. 3. counter without blocking by moving out of the line of attack and striking him.