The second serve (part one)
A strong second serve is perhaps the most important element in developing a competitive game. The true strength of an opponent lies not in the speed of their first serve, but in the strength of their second serve. Once you start playing against good players, you cannot simply push in your second serve - they will hit winners off of those shots every time!
It is critical to practice the second serve a lot - even more than the first serve. It will serve you well during important moments of each match.