Why you're not getting bigger or stronger
Every gym has certain people who train a several days a week, every week, without fail. They come in at the same times, do the same exercises, same reps, same weights… they're the most consistent trainees in the gym. But they never get any more muscular, stronger, leaner or anything except older. Why? They're TOO consistent. If their workouts never change, why would their bodies?
Our bodies are designed to adapt to new conditions or stimuli. In the past, before humans became so technologically advanced, this was necessary for survival. Now that survival isn't such an issue, this adaptability is necessary for more important things like bodybuilding. You see, when you regularly lift heavy weights, your body notices that it's under quite a bit of stress, which it doesn't like. So to deal with it, your muscles become bigger and stronger so next time you go to lift those weights, it won't be such a stress to your system. Similarly, when you run massive distances regularly, that is a significant stress to your body, so your body needs to adapt. This is a different kind of stress to lifting heavy weights, so your body responds differently, by making your muscles puny and pathetic.
The common thread here, and the point of this article, is that in order to force your body to adapt the way you want it to, you need to put it under stress or "overload" it. Not only that, but you have to keep putting it under stress as long as you want to continue the adaptations. You may start out lifting 10 reps with 50kgs in the bench press, and it may be really hard. You stick with it, your body adapts, your muscles get a bit bigger, you get a bit stronger, and now 10 reps with 50kgs isn't so hard anymore. If you keep using 50kgs for those ten reps, you won't get any bigger or stronger, because your body doesn't need to. What you need to do is increase the training load to put your body under more stress, and continue adaptations. This is “progressive overload”, and there are four ways to do it:
1. Increase the weight
This is the simplest and best method, in my opinion. Last week you did 5 reps on the bench press with 50kgs, and this week you did 5 reps with 55kgs. That's a 10% improvement. Sometimes you'll have to drop the reps a bit to increase the weight, such as 5 reps with 50kgs one week, and 4 reps with 55kgs the next, but that's no problem - just aim to get the full 5 reps with 55kgs the next week after that.
2. Increase the reps
I mean total reps here, so suppose one week you do 4 sets of 5 reps (4×5) with 50kgs. Keeping the weight the same at 50kgs, if you do 5×5 the next week, or 4×6, either way it's an improvement because you did more total reps. This is good, because it means you're doing more total work.
3. Change the time
here are two parts to this: timing of your sets, and timing of your individual reps. You can make a workout harder by decreasing rest intervals and decreasing the total workout time, but using the same weight, sets and reps. This means that you're completing the same amount of work in less time, which increases your intensity (and also gets you out of the gym sooner). You can also make your reps take less or more time, which strangely enough can make it harder either way. Put briefly, you can aim to complete your reps more quickly and explosively (lower the weight with control, then lift it as fast as possible), which is good for improving strength. Alternatively you can aim to complete your reps very slowly (e.g. count to five on the way down, pause, count to five on the way up) which increases your “time under tension”. This is good for hypertrophy (increasing the size of your muscles).
4. Change the mechanics of the lift
By changing joint angles and lever lengths you can make lifts harder or easier. For example, in the bench press, you can keep the weight and reps the same but make the exercise harder by taking a very narrow grip on the bar. I don't advise this as a method of progressive overload because the changed biomechanics of the lift emphasize different muscles. So if you want to target the triceps, use the close grip bench press, but if you just want to make the bench press harder, use the advice in points 1, 2 and 3.
If you keep making your workouts harder using the techniques above, and eat enough good food to recover adequately, you will get bigger and stronger… for a while. Then you'll need a new routine.