The summer is a great time to educate yourself and begin to incorporate mental skills into your athletic performance. Take the time to browse books and manuals about mental training/sport psychology in order to get a firm understanding of mental skills. There are a number of publications available that focus on specific sports, some for coaches, others for athletes, and all about the mental training. Find books/articles/manuals that interest you and apply to your sport(s). Once you've studied up and have a working understanding of how sport psychology can help your athletic performance, use this summer to practice implementing your mental skills.
Incorporate these skills into your physical program this summer. If you are not in a summer/travel league, this will give you the opportunity to learn how to use mental skills before you have to use them in competition and practice. As with physical skills, mental skills need to be practiced and refined.
Again, this is a pretty cool way to get some summer gains mentally while preparing for the upcoming season. Or, as always, you can ask me to help you with all this and I'd be more than happy to!