Even Arnold Schwarzenegger said that one has to surprise the muscle. It is absolutely true! If you go to gym 5-6 times a week, in a fortnight your body will get used to the set of exercises and there will be no more desired effect. Sometimes, it is enough at least to change the position of your body when you are doing exercises, add some more weight or even amend the sequence of movements.
My choice was to have different workouts every day of the week I go to the gym. And in order to surprise my muscle even more, I almost never have the same sequence or exactly the same set of exercises as the previous week. Thanks YouTube and Google, there are thousands of different videos and articles that can diversify the set of physical activities in the gym. (if you are lucky enough to have a Windows Phone, you can download a wonderful WorkOut app that will provide you with various types of workouts)The only that never changes is my Cardio! At least 20-30-40 minutes of cardio for warming up and starting up metabolism.
Here is a video from which I include a lot of parts into my gym routine. It is 22 minutes long but it is very intensive. That is why it is good to include at least 4-5 types of the sets offered in that video and start using them in the workout routine.