To be most successful in strength, speed and explosive training for football you must continually increase the intensity of your workouts. There is no getting around this fact.
In weight training you must regularly increase the amount of resistance that you are using, especially for linemen. If you are in the backfield, you must also train explosively to run faster and to be quicker after developing a strong strength base.
However, many players and coaches are abusing these recommendations to the point where they are causing injury and poorer performances. Because of this it is important that you understand how to safely build intensity and how to best use it in your workouts.
Before starting to use heavy weights your body must be prepared to handle the stress. In general, your muscles develop at a much faster rate than your ligaments and tendons. Thus, if you start using very heavy weights too soon (or if steroids are used), your muscles may be able to handle the weight but your tendons and ligaments may give out and be injured.
I attribute this last factor to the many injuries that occur in the weight room and on the field. Because of this you must first develop a strong physical base before doing high intensity work. To do this use exercise regimes with “low” weights and higher repetitions which best develop the ligaments and tendons.
When using very heavy weights the movement involved is slow and your nervous system learns slowness. If such workouts are continued long term, they transfer to your performance on the field. Instead of being quicker, more explosive and faster, you actually become slower in your movements and your performance effectiveness decreases.
To prevent this from happening you must do speed and explosive work (speed-strength work) after developing a strength base. In other words, you must figuratively convert your strength to speed and explosiveness. This is the key to success in football and in other sports that require speed and explosiveness in skill execution as, for example, in acceleration, running, throwing, kicking the ball, leaping for the ball, etc.
You should gradually increase intensity in your workouts until the season starts. There should be very little weight training in the range of 90-100%, and most in the 75-85% of maximum zone and some in the range of 65-75% of maximum, coupled with explosive and sprint training. Such trainings should follow a progression based on an adequate strength and conditioning base.
Doing only one kind of training year-around or in the off-season will not enhance your playing. You must lay a foundation to do the necessary work that must be done for success at a particular position.
For more information on training programs and specificity of training, see Build a Better Athlete and Explosive Running