There's a misconception that the best way to practice for tennis is to perform solely "standard" side-to-side drills. This is certainly a good drill for learning proper movement to the ball. However, as very few points are in fact played in a "coast-to-coast" fashion (i.e., sideline to sideline), every practice should seek to incorporate an inside-out drill as well (that is, forehand from the backhand side, backhand from the forehand side). Inside outs assist the player in squaring the leading shoulder to the ball (a particular problem with players with extreme open stances) and staying on their toes. It is important to note that in tennis, running TO the ball is as important as running AWAY from the ball. Inside-out teach the player how to properly set up for every shot in order to maximize power, accuracy and positioning.
Initially, the coach should feed "regular" side to side sets followed by inside out (e.g. sets of ten each). The player's goal should be to maximize power and placement in order to generate the best angle that she can under the circumstances. After the player becomes proficient at the fed drills, the practice can incorporate the "corners" drill where the coach stays in one corner and moves the player around side to side (the player hitting one "regular" shot followed by an inside out shot). For tougher practices, the player can hit ALL inside outs, although this would depend on the coach's fitness and expertise. A great drill is the typical figure 8 drill but where one player has to hit inside outs (cross-court). Similarly, a more basic drill is the "lazy 8s". This is where the players play cross court and HAVE to alternate shots regardless of the bounce (i.e., if the player hits a forehand off the feed, the next one MUST be a backhand). As the name implies, this is a variation of the standard figure 8 drill although the court is smaller and footwork is more focused. This is a also a great warm-up drill as it keeps the feet moving.
Learn to master the inside-outs (strokes and movement) and your angles will be deadly.