Adapted from my classroom:
A marble jar. My elementary school kids warned a marble for good behavior, utilizing life skills/morals, excelling in whatever area I saw fit or that was Pre-defined. When the jar filled we had "fun Friday" afternoon.
In gym it works great. You can reward a marble for anything. Acquisition of a new skill may earn multiple marbles, high level of effort, reaching a goal, etc.
Once the jar is filled up the team is rewarded as all the girls contributed to filling it. I usually Pre-define the reward- sleepover or no conditioning 1 workout or games for warm up... You name it. I've even written several rewards down & we picked out of a hat what the reward was.
I like this reward system as you can motivate an entire group by handing out 1 marble to an athlete. They all want to earn one!
I have had success with all ages- 6yr olds & 16yr olds enjoy putting a marble in the jar
Trick is finding a decent size jar that will be filled with enough hard work marbles that it is worth working for... Doesn't reward too often, but often enough they strive to fill it!
Mine takes a couple months to fill generally-depending on their work ethic & how liberal I am rewarding marbles!