As far as variation goes, there is no one rule, principle, or routine that is going to give you the results you want. You have to listen to your body and find out how it responds to certain exercises and training stimulus. You have to also consider your metabolism and calorie expenditures. If you're serious about exercise and developing your fitness level then you have to consider everything.

I've been lifting for about 4 years now towards bodybuilding and I power lifted in high school for two years. I had a few national records and was the top lifter in my weight class. I've learned that your body will adapt to stresses applied to it very easily. I have several routines for different training cycles, whether it be mass or cutting or a strength routine. In these routines, you have to alter them a little every time to shock your muscles. Nothing too drastic, just change the exercise order, or use a different training principle. You do not have to go and do a hundred sets of one exercise to shock your muscles. Sometimes it might be your diet that is hurting your gains. If you want to achieve the results, you have to put the time and effort in. No one said it was going to be easy, but if you love being fit, then you can make a few sacrifices to get the results you want.