Weightlifting is a rare activity in which the more you do it, the worse you can actually get at it. But this is a complicated claim so let me explain what I mean. Your body likes to minimize stress and pain any way it can. And the more you do an exercise, the better your body becomes at adjusting itself to relieve the stress of that exercise.
You adapt to do the most work possible, using the easiest way possible, and in most cases this is ideal, weightlifting just isn't one of them. In weightlifting the idea is the stress the muscle as much as possible, as quickly as possible.
The more your body learns to adapt to that exercise by using other muscles, or leaning to one side or another, the less it hits the target muscle and the harder it becomes to build that muscle.
Does strength and additional muscle mass go together? Strength occurs when your body adapts and learns the best possible leverage to preform an exercise. Therefore you will become stronger by practicing an exercise as your body adjusts the weight off the target muscle and onto other additional muscles, this is obviously not the aim of a bodybuilder. The goal is stressing the muscle. Machines can help with this situation. Free weights allow your body a lot of room to adapt and use itself for leverage. Machines offer a much firmer form and make it much harder for the person to "cheat." Maybe this will open the mind of those who swear by free weights to at least give machines a chance as they do as they are meant to do, they remove the secondary muscles and hit the main muscle harder.
Frequent routine changes can help keep your body on check and unable to adapt to your exercises as quickly. I have worked out the same way for the last few years and I have seen great results with it, I have also read about pros using the same method I do, even though I do not believe in using routines of the pros. I was already using this simple method when I read of others doing the same, it goes like this. When I wake up in the morning I know what body parts I am going to do. But not until I step foot in the gym do I know which exercises I plan to do.
Shocking The Body
This is great for me for several reasons, one it keeps my body wondering right up until the last second what it is in for, it cannot prepare for what I am about to do to it, and second (and nowhere near as important, but an added bonus) my gym has somewhat limited equipment, and this schedule means I never have to wait for any equipment and let my muscles cool down as I can jump around the room as I need to from exercise to exercise.
I read articles stating that it is a good idea to keep your routine the same for around a month otherwise you'll never know your strength increases, and your body won't have time to grow stronger on any exercises. I don't quite believe in that theory, nor do I believe that strength increases are really necessary to grow muscle in the first place. As was discussed above, getting stronger can merely mean your body as learned to use leverage better, and building muscle is just keeping the weight on the desired muscle, which is difficult to do; your body will do whatever it can to stop it.