Well, for athletes that I will generally have most of the year for their offseason with, I use annual planning principals and we train with different phases/cycles throughout the year.
Looking specifically at immediate post season, the most important factor is recovery. If you go right into hard and heavy training, bad things are going to happen. Competitive seasons take a toll on athletes, especially high school athletes. Take into consideration the fact that many high school athletes participate in extra sport competition during their "offseason," and it becomes even more important. (Ideally, there should be an entire offseason dedicated to work outside of the specific sport to avoid burnout).
So what do we do during this period of time? Well, we train at an easy to moderate difficulty level and put a focus on general and corrective exercise in order to iron out any imbalances that were created during the season, give the body a stimulus in order to prepare for the harder training to come and to facilitate recovery throughout the body. Here is an example of a workout that one of my baseball players went through today.
Soft tissue work/Mobility Drills to warm up
Wall dribbles for cuff activation
1) Db split squat 3x10
2) Split Stance Cable Push 3x10
3) One leg RDL 3x10
4) Split Stance Cable Row 3x10
5)High to low Face Pull 2x12
6)Half Rotating Side Bridge 2x8 each side
7)Scap Raises 2x10
Specific hip and shoulder stretches to finish
All of these exercises were done plenty reps short of failure wit h light to moderate weights and around 45-60 sec rest between sets. I like to use plenty of unilateral movements during this period to fix any strength imbalances that were developed during the long season. In this case, specific shoulder and scap exercises/drills are included for this athlete's individual needs. This particular athlete will be training twice a week in the weight room for the time being with some minor baseball stuff going on a couple of other days during the week. His other workout will involve mostly general total body conditioning work such as sled dragging for upper and lower body, jump rope, bodyweight drills, ab and hip work, etc.
This phase is usually known as anatomical adaptation/active recovery and is very important in ensuring that athletes don't burn out yet maintain fitness levels and prepare to increase fitness levels in the near future.
Recovery is often an underrated aspect of training for the general population as well. Recovery is when you get better and stronger. Without recovery, your progress will go nowhere. Nobody can go all out all of the time. That is only asking for disaster. Active recovery is the most effective form of recovery in my opinion. This can involve things such as soft tissue work in the form of foam rolling or massage; it can include contrast showers or hot tub soaks; it can include light mobility and bodyweight drills to faciliate blood flow and nutrient delivery; or it can include playing a physical game or sport outside of the usual practice of the individual at hand. This goes for general population and athletes alike.
With an average fat loss or general population client, I usually design programs in 4 to 6 week progressions. With a 5 week progression, I will usually use something like this.
Week One- Intro Week- Focus on technique and movement quality
Week Two- Raise intensity and train hard
Week Three- Raise volume and train hard
Week Four- Build on the last two weeks and train moderate to hard
Week Five- Deload week- Train moderately
Week Six would then be the beginning of a new program.
This is not the end all be all of progressions that I use but it provides a decent general outline. It is during those deload weeks that the body has a chance to supercompensate and get better and stronger from all of the hard work that has been put in.
Recovery might be what you are lacking if your progress has stalled. Don't be afraid to go easy from time to time as it will pay off tremendously.