Which of these are you guilty of performing?
Specializing in a sport and being entirely unwilling to deviate: Variety is critical for injury prevention, mental outlook, and general health. Unless you are a triathlete, pentathlete, or decathlete, you might find yourself unwilling to stray from the single sport you know. First off, you should strength train. Strength training is no longer an option; it is a requirement. Crosstrain before you get hurt (and you might not get hurt in the first place). Try something that is non-competitive and don’t turn it into a competition. (Yes, that’s probably hard.) Take a yoga class. Play pickup basketball with a couple friends or just shoot around. Try swimming. Just do something that is very different than your normal and do it often. This is especially true for young athletes who aren’t finished maturing. Research suggests that athletes who didn’t specialize at a young age can perform better as long as they were performing some form of athletics. The key for all of us is simply being an athlete.
Assuming your skills and technique don’t need further work: Skill work is primarily about training your nervous system to use a specific pattern. A good overall athlete has more options for movement patterns. Some patterns are highly ingrained and some are not. You want instinctive patterns to be close to an ideal. That way, when fatigue occurs you still demonstrate precision and efficiency. Every athlete could improve their performance with at least weekly emphasis on movement drills, strengthening specific motions, increasing muscle power output, and basically fine-tuning they way the nervous system creates each sports-specific motion. Regardless of sport, there’s room for greater efficiency and adding skills to your repertoire. Even running, which some people assume is innate, is a skill that should be broken into components for drill work.
Training alone all the time: Sure, you are probably mentally tougher than the average bear, but this problem allows you to slack off occasionally when there’s no one there to push you. Which means you drop into the dreaded moderate efforts that lead to “dead” muscles, overtraining, and slower than optimal nervous system patterns. My motto is “there’s always someone faster and stronger.” Go train with that person at least once a week. A coach could assume part of the role of a training partner, so I’ll give that half an exception.