Never Play Your Opponents
One of the biggest failings I see in many teams is the handing out of disproportionate levels of respect to opponents. I see it all the time.
In reality, you are not playing your opponents when you are playing a tennis match, you are playing the ball!
First and foremost, you have to react to where the ball is going, what the ball is doing and what you are doing to the ball before you worry about your opponents.
When you are playing a highly ranked pair, you need to respect their standing, but not so much that it stops you from playing to win. See this as an opportunity to go out and play as hard as you can.
View it as a chance to put together everything you've been working on and see how it matches up. Even if you end up losing, the next time you play a similar rated/ranked pair you will be that much stronger for the experience.
When you are playing a lowly ranked pair, this is another great chance to move your game on.
Try to use this opportunity to try out some of the things you have been trying to introduce to your game. Implementing new tactics during a match is always more beneficial than doing it in practice—and there won't be as much pressure in a situation like this, so you can play with more freedom.
Toughness Through Practice
You don't just wake up one day mentally tough!
Mental toughness is something that needs to be learned and honed regularly. It is a skill like most other skills that is susceptible to the training law of reversibility—you either use it or you lose it!
So you need to place yourself in pressure situations on a regular basis.
These experiences will make you stronger and will also give you the edge over those that haven't experienced the things you have.
It's not quite like practicing a forehand, which you can to a certain extent just hit 10,000 balls to improve and groove the motor response, mental toughness comes from the practice of being, well, mentally tough.
So, seek out as many opportunities to challenge yourselves mentally as you can. You will learn a lot about yourselves and your coping mechanisms as well as giving you the much-needed practice that will be needed when it matters the most.
Practice Tip - incorporate pressure into your practice i.e. play a set where you as a team only have one serve on service games or start each game 30-0 down etc.