Reward drives human behavior. Undoubtedly, fitness is a behavior. Ultimately, we'd like athletes' behavior to be driven more by intrinsic or internal rewards than by external sources, but this type of thinking takes time to develop.
So, what can we extrinsically reward in terms of fitness behavior? Here's a place where the prudent coach really has to put some effort into thinking about the big picture. The trap can be focusing rewards mostly on the amount of weight lifted in the weight room, i.e., "pound clubs." Ok, but rewarding solely these efforts has some down side, like injuries and lack of execution just to earn praise from lifting a weight.
Personally, I try and find ways to reward attributes such as commitment, effort, reaching goals, and team-work; more so, than poundage. Not to say that pounds aren't important, but what's the carry-over value outside of the weight-room? Certainly something to consider.
One reward that has a lot of value is simply telling an athlete when they're doing a good job, especially if the praise or reward is spontaneous and relates to an action or behavior that can be identified. However, it can be so easy to focus on what an athlete doesn't do well and that fosters negative reward. I'm not saying that trying to be an athlete's buddy is the way to go because that's another trap too, but offering verbal praise has a lot of benefit.
An aspect of reward that is certain, lack of a rewards system will result in a reward vacuum. Something is going to fill the void, usually bad information from other athletes. So, be proactive and identify as a coach what behaviors to reward, develop a rewards system, and find creative ways to help athletes learn how to transition rewards from external to internal. This is groundwork for helping an athlete to develop emotional intelligence.