Since most points end after the serve (ace, service winner, missed return or double fault), it is extremely important to get your return of serve in play and force the server to hit another shot. Focusing on the basics will make this more likely and will also give you a chance to get the server in trouble.
The two most important keys to a good return are hitting the ball early and making solid contact. Watch the ball all the way from the server's hand, coming off of their racquet, and through the point of contact on your side of the court. Once you identify if the return is a forehand or a backhand, use your shoulder turn to help bring the racquet back and keep the backswing slightly shorter than a regular ground stroke. Try to keep things simple since there is a shorter time between hits on the return. Your follow through should be similar to a regular groundstroke unless the serve is so good that all you can do is block it.
Another good idea is to pick a spot to hit the return before the ball is served. You can pick the same spot for both your forehand and backhand or you can pick two different spots depending on which shot you are hitting. I always liked picking a spot that is in the early direction since most misses on the return are hit late.
A good goal for your return of serve is to make more returns than the number of points your opponent wins on his serve (aces, service winners, forced errors, and unforced errors). This should give you a good chance of winning more of the return points and breaking your opponent's serve on a regular basis.