I asked that very question to my team and got a variety of replies that didn't satisfy an answer that would always answer this question. For example, some would say "hit more errors" or "make less first serves" but that doesn't answer the question. Andy Roddick learned this the hard way in the 2009 Wimbledon final against Roger Federer. The answer is simply, a tie-break. If you can hold serve every time and assuming your opponent does too, you are guaranteed a shot at winning the set in a tie-break. As many of you know, anything can happen in a tie-break.
What I'm implying here is how much effort do you put into practicing to hold serve. I'm not talking about just serving but the act of planning out the next couple of shots after the serve to give you the best shot at winning. I see a lot of players serving for practice but I would encourage you to take it a step further with your training. Try to serve with two balls in your hand and after serving the ball, throw the second ball up to imitate a weak return. Change your grip and hit the second ball hard in the spot you feel is giving you the best chance at winning. You can do something similar with a practice partner. Give yourself three shots to end the point on your serve, otherwise your practice partner gets the point. The idea is to practice being aggressive and not allowing your opponent to feel comfortable in a rally while you are serving. This isn't going to happen all the time but practicing aggressiveness after your serve can pay off in the long run. The chances of winning big points increases dramatically with the person who is proficient in aggressive tennis.
The next time you practice your serve, consider what you can do to work on holding serve rather than just hitting targets. Happy hitting!
Having trouble with high balls at the baseline? Try these tips out with this video: