Here is some common mistakes that we mustn't do...
Not warming up. The warm up is not only about injury prevention. A good warm up improves your performance by mobilising your key muscles so they can be used both more quickly, powerfully and with better timing. That means a good 15-20 minute warm up is vital before every session or match if you want to get the best from your skills.
Warming up badly. Almost as bad as not warming up, a bad warm up can slow you down and cause injury. That means making sure you warm up for long enough before you pick up a bat or ball and don't do any static stretching (as it reduces performance).
Playing without a goal. Going to nets or a game without a goal is not a very effective approach. It's far better to know what you are aiming for. In tests, the people who did the best were those who set goals compared to those who just tried their best.
Avoiding fitness. In the UK most people equate fitness training with boredom or pain. Neither needs to be true, despite what PE teachers might have put you through as a school kid. Good fitness training is both enjoyable and vital for your game.
Being too specific. Cricket is a game of skill, but just practicing cricket skills will not improve your general sport skills. That means vital factors like speed, agility, power and mobility are left to deteriorate, and with it your cricket ability.
Not being specific enough. It's vital to develop both cricket and general sport skills. It's counter productive to train in ways that are not related to cricket at all. So leave the long, slow paced runs alone and keep heavy weight lifting to an absolute minimum.
Not drinking enough. Dehydration is a major cause of performance loss. However, hardly anyone drinks enough in training or play. It can be difficult to do, but aim to drink a small amount every 30 minutes during play with 1-3 litres in a day.
Staying the same. Different times of the year call for different approaches to training. If you train the same all the time your body and mind will get used to the training and you stop improving. Also, you may be putting too little or too much strain on your body without a periodised approach.
Doing too much. Younger club players can easily play cricket every day in the summer. Older players can be playing 3 times a week, going to the gym and training with the team. It's almost as easy to do too much as too little. So make sure you have a rest at least once a week.
Giving up after failure. If you drop a catch in the slips you may tell yourself you are a bad slip fielder, if you are not getting results quickly in the gym you pack it in. In fact, failure is an indicator you are learning. Good players keep trying.
Not sure if that helps you at all though Jake.