Here is a neat drill for practicing two things: returning solidly away from the sidelines (thereby neutralizing the server's initial advantage) and, for the server, learning how to construct points without relying on aces and service winners. The fact that the server is also trying to improve his jam-serve is a bonus (since this is often under-utilized in junior tennis).
The drill is as follows: set up an area in the middle of the service box that is 2.5 feet from the [service box] sidelines (see shaded service box area above). This will give the returner ample opportunity to "see" many serves and get the balls back in play. Also, set up an area on the server's side that is 3-5ft from his sidelines. The returner must return in this area in order for the point to start. After that, anything goes (i.e. players can use the whole court). But the purpose is to train the returner to hit smart, high-percentage shots on the returns and apply lots of pressure on the server. This is also good for the server (especially if he's the big serving type who relies a lot on quick points) since now he has to construct a point with his head and his heart. Play practice games up to 10, 15 or 7-11 (I.e. Where the server must get to 11 before the returner gets to 11) and rotate; rinse and repeat as necessary. With lots of opportunities to see the serve and practice returns you, as a returner, will become more proficient at putting some heat on the server. As a server, you will gain more confidence from knowing that you can construct a point without relying on aces or service winner (allowing you to actually be more relaxed on your first serves). If possible, have two or three servers against one returner and play two points against each. This way, the returner gets a "look" at more than one service motion and can adjust for each.