Manipulating your routines and diet in my humble opinon are the keys to constant muscle growth.
I think you should follow a routine until it no longer works for you (most people prolly between 4-6 weeks). I think it is a bad idea to change often, because it`s hard to tell if you are truly progressing (unless you are adding a pound or two of lean mass every single week.)
What I do is write out a program and just fill in the blanks when I get to the gym (the exercises how many sets, the tempo I will be follwing), I make sure that I do the exact same workout for a few weeks in a row (at least), this way you can tell if poundages go up...and we all know that equates into progress (muscle or strength depending on your goals). Of course your diet and rest must also be adequate.
Remember, if you fail to plan, you plan to fail.