Many players who are trying to improve their game go out on a daily basis and hit balls--literally, they just hit tennis balls.
This type of practice can be helpful when trying to develop muscle memory, time and rhythm, and, basically, to groove your strokes. There is a place for this but if you really want to improve your game you should always walk out on the court with a specific shot, strategy, or idea in mind that you will focus on. Simply rallying from the baseline will not give you the same type of ball or situations that you will be faced with in a match.
It is important to do a lot of hitting in a live ball situation (players exchanging shots back and forth) with a competitive idea--try to win the point eventually. This will give you a similar feeling to what you deal with when you play an actual match. Of course, it is still a less stressful situation with not nearly the pressure, so it is best to get as close to a real match as you can.
Playing practice points, sets, or matches with a specific idea (keep all balls deep at the beginning of the point, for instance) will give you the closest replication of an actual match. Playing tiebreakers with your idea is also a very good way to practice dealing with the pressure and feelings you encounter in your matches.
Challenge yourself to practice with a purpose and simulate the actual points you would like to play in your competitive matches for better results.
Steve Annacone, USPTA Elite Pro, is the Director of Annacone Tennis (http://www.annaconetennis.com) and MyHamptonsPro (http://www.myhamptonspro.com). For details on lessons and clinics throughout the Hamptons contact firstname.lastname@example.org or email@example.com or call 865-300-7323