Mental toughness and physical toughness go hand-in-hand. One simply cannot be mentally tough without first developing his physical characteristics. All the best players out there have a "gotta want it" attitude. Whoever "WANTS IT" the most, usually wins. This person never gives up in the face of adversity.
There is no easy and straightforward way to develop mental toughness although one drill that has worked for me (along with other players) is to place a physical exercise component prior to playing 2-3 sets. For example, players can do a timed 2-3 or 5 mile run (depending on level of fitness and age) PRIOR to playing a 2-3 set match. The player who wants it the most will usually win. With two players of equal will-power, both players will improve dramatically since they are forced to dig deep and develop a "never say die" attitude. If a long running course is not available, player can run on-court sprints for 30 minutes (or jump rope) and then play.
The key to this type practice is for the player to figure out ways for winning points most efficiently and to focus and fight for every point. Physical fitness begets mental toughness. This is also a useful drill given that tournaments often have back-to-back matches with 30 minute break in between. You can pull through a tough 3 set match against a seed in the first round and than have to grind out a match against a lesser player in the afternoon. A player needs to be tough enough to believe that he has done everything possible in order to defeat both opponents back-to-back.
Other practices can involve playing sets without "proper" warm-ups. That is, the players just show up and start playing points. Often times, particularly at junior events, the players will not have a proper warm-up anyway due to lack of hitting partners or tennis courts (or traffic that precludes the player from showing up on time). So it's important for the player's to be ready from the word "GO".
In addition, players can play sets but switch rackets with their practice partner. Often times, the player will break all of his/her strings during the match and be forced to finish with a replacement racket. Similarly, vary the location of the practices - sometimes at the country club, other times at the local park, maybe someone's back-yard, indoors, clay, grass, carpet, etc. MANY junior tournaments (and even open or future events) are played at facilities that are comparable to public parks. Many are in fact played AT public parks. There will be people playing soccer on the adjoining field; families barbecuing; cars backfiring; and dogs barking/fighting. These are conditions to which the player must become accustomed. Until you reach the manicured lawns of Wimbledon, it's mental and physical "sink or swim."
A coach should not always tell the player in advance where you're meeting her. The purpose of these practices is to develop confidence that the player can win under any conditions on every court. Champions are made on the back-courts not stadium court so no need to practice there. Regardless of what the opponent or fate throws your way, you can figure out a way to pull through.