A majority of the fitness population have heard the same advice when it comes to strength training. 3 sets of (some amount of reps), why not 4 sets? why not 2 sets? Lets talk numbers.
Take a few minutes before lifting to figure out what area of your body you want to improve that day.
Get to the gym and warm up with body weight exercises and stretching (10 minutes).
Pick the major muscle groups you're going to be hitting and start warming them up with light sets of movements that work that specific muscle group. Doing these light sets are important and will reduce the possibility of injury.
Since you have hit the major muscles, your secondary muscles are now warmed up and ready to be focused on. since they will be partially fatigued at this point but still feeling ready for work, hit them hard. Drop sets/ ladders/ pyramids/ partial rep sets with light weight burn sets/ heavy low rep sets.
now that you have hit the major and minor muscle groups to be focused on. hit some of the weak points you missed, calves, forearms, core, lower back.
At this point you should be pretty well roasted and praying that it ends soon. This is the point where you can walk out, or keep going for that extra mile.. literally. Follow up your workout with intense cardio, (double unders, interval sprints, sprint a mile, high box jumps, tabata style interval training".
Now that you have put in all the possible energy you could expend. Be sure to spend the next 30 minutes and eat and drink. Consume the amount of calories you need to replenish or eat slightly less if your looking to lose weight. Water is just as important and food, drink plenty of water to ensure you aren't dehydrated.
Workout
21 - 15 - 9 Reps
Wall Ball Throws ( 20#, scale down to jumping air squats)
pushups
Between each round, run 400m with wall ball, or 10# weight
Go hit the gym!