Set Your Expectations High for Success

At the level of 4.5 tennis and below, success in doubles is about two things: serves and returns. Success in singles is about rallying and finishing. In other words, in doubles, if you can serve effectively and get almost all returns back in play, you will beat most other teams at your level. Your other shots come into play, yes, but you can be mediocre with those if those first two are extremely solid. In singles, the returner has a lot more freedom to get the ball back in play and there's not a guy at net, so longer rallies are much more common. You have to be able to sustain long rallies, and when your opponent makes a mistake, you have to know how to finish. So, to sum up:

Doubles: Serving and Returning
Singles: Rallying and Finishing

The first step in achieving mastery of those skills is to set your expectations extremely high. The following phrases sum up the mindset:

Doubles
Serving: "Nobody breaks me."
Returning: "I don't miss a return."

Singles
Rallying: "I'm gonna hit 50 balls."
Finishing: "I am going to end this point."

The importance of setting your expectation high is to motivate you to reach that goal. The expectation should be with you at all times - especially during practice. If you practice or play and accept failure, you will continue to perform at the same level.

When you fail, the expectation needs to be aggravating - not in any negative way, but in a motivating way so that you know what to do to not fail in the future. This is a very important point. Many tennis players get mad when they miss a shot, but they make several mistakes with this anger. First, they let their opponent see that they are emotionally out of control. This energizes their opponent and sends even themselves a defeatist message. Second, they let the anger feed into negative thoughts like "I suck." or "Why me?" I'll answer both those statements. You don't suck, you just made an error - now figure out WHY you made that error. Why you? Because you are in control of your own errors. Don't forget that - you are in control of your own shots and the opportunities you give your opponent.

So, again, the important point is your high expectation should put you in an aggravated state when you don't live up to it, but that aggravation should lead to positive action. For instance, losing a match because you couldn't hit 50 rally shots should motivate you to practice your groundstrokes for hours until you can. Or, getting your serve broken in doubles, should motivate you and your partner to carefully examine why you lost that many points and make adjustments in the next service game to avoid those same problems. Or, missing an approach shot should motiviate you to figure out exactly why you missed it - was it the right shot selection, did you decide what do do quickly enough, were your feet in position, was your racquet ready in time for the stroke?

This aggravation should be internal and any external display should be one that indicates control and thought. No negative body language. No whiny tone. If you realize that you missed the shot due to poor footwork, you should at least say "Feet!" in your head and smile. Why are you smiling? You're smiling because you know why you missed that shot which means you are less likely to make the same error the next time. That means you are in control and a very dangerous opponent.