"Leap, and the net will appear." – John Burroughs
It's possible that your athletes are as strong as you want them to be now and you feel that they won't benefit from any additional strength.
Now that we got the joke out of the way let's get on with it.
- Use the correct working range for reps Generally, your athletes should not be doing sets of less than 5 reps or more than 30 reps. 7 - 15 reps is a nice workload for much of the lifting you will do. I like one warm up set and one work set. On the work set once you can get 15 reps easily, add a bit of weight at the next workout.
- Add single limb movements. Don't go heavy on these. The main thing is that the weights put the body in an unbalanced state. The sports your athletes play don't present athletes with forces that are all nicely balanced and come at the athlete perpendicular and centered. So, it makes sense to do some movements that cause the body to have to stabilize itself.
For legs consider step ups with a dumbbell held in one hand. You can alter the hand that holds the weight doing two sets per leg. One set with the weight in the right hand and one set in the left. Single arm db overhead press is a good lift. If you have resistance bands, these are optimal for this type of training. Single arm standing bench press is a great move for linemen in football.
- Cut down to three lifts per strength session. The three simplest would be a lower body movement, upper body pulling movement and upper body pressing movement.
- Focus on technique. Slow down the lift so you are doing a 4 second eccentric and 4 second concentric. This will eliminate any momentum and keep tension on the muscle the entire movement.
The athlete should feel the muscle being worked very directly. They may need to use lighter weights if they've been moving faster and cheating the weights through sticking points.
- Drink some water. These days lots of people either don't get enough water or even if they get enough liquid aren't getting enough pure water. An extra quart of water consumed throughout the day over what they athlete is currently doing will help with recovery from training.
- Focus on a weak spot. Athletes tend to like to do what they are good at, as a coach it's your job to spot what they are weak at and work to improve that. Find their sticking points and what muscles are weak. Don't add volume to the workout to fix the weakness. Instead drop the primary lift with the weakness every other session and do a specific exercise to work the weak muscle.
- Talk with your athletes and learn about them and what makes them tick. The more you know about each one, the easier it will be to motivate them in the weight room. The motivation will lead to better gains
These are seven ways to strengthen your athletes, but really this is a starting point. Of the seven I've outlined you could break each one down further. You could come up with 7 ways to motivate your athletes for instance.