The key to working out effectively is intensity. And when I say intensity I mean all different ranges of intensity. This must be coupled with appropriate volume and by this, I mean how how many repetitions or sets of a particular exercise you are going to do or how long are you going to be doing said exercise. Changing both of these parameters on a regular basis and getting adequate rest is essential for effective fitness. How do I know this? Well, there are a lot of very smart guys and gals in the fitness and strength and conditioning world and I have read a lot of what they have to say. I've also had the pleasure of training and coaching a number of athletes, fitness participants, and fitness enthusiasts over the years and I have found that a lot of the principles that I have read hold true at the end of the day.
I have taught kettlebell classes, coached at a crossfit affiliate, coached a girl's weightlifting team, and personally trained a variety of folks using a variety of methods. These are the lessons I've learned so far:
Diet, exercise, and rest are synergistic. This means they work together and rely on one another to be optimal. If these three elements are not optimal, you will not get stronger, lose bodyfat, and or perform better. All three must be in place and more often than not, at least one, if not two are completely ignored. I have coached a number of people who are strong runners, swimmers, and workout six times a week, and yet, they cannot lose weight, increase their deadlift, or get a real pullup because they won't eat enough protein or take enough rest.
Things do not have to be difficult to be effective. This applies to everything. There is a learning process to every new thing we try. Its not just a mental process, but a physical process as well. Our bodies must adapt to become more coordinated. Nerve impulses from the brain to the muscle are modified to allow for more precision and self-awareness. The muscles, tendons, and ligaments all need time to adapt to the forces exerted by new movement patterns. This means that even at a low level of stimulation, change is occuring. Don't miss out on the subtle changes that will improve performance and maximize results down the road.
Just because a workout is "hard" does not mean it is "good". If workouts only had to be hard to be effective, well, I'd have a much easier job. I can make anyone tired, sweaty, and possibly nauseous. However, using those guidelines, I can't guarantee that you will be injury free six months from now or that you will have met any of your fitness goals. Patience is a virtue. Practice it.
Change is good, but too much change is bad. Changing intensities over the course of a week is good. Changing your training focus to work on one or two aspects of your general fitness or goals every two to three months is good. Changing your routine regularly to keep boredom at bay is good. Having completely random workouts and using such a broad range of techniques that you can't adequately practice them all is bad. It may work for some folks, but its not ideal. Pick the ones you are good at and keep them as your core set of movements. If you want to develop another discipline such as yoga, kettlebell technique, or olympic lifting, try devoting a focused period of time to practicing and learning the techniques to see if they are something you want to pursue and/or use as an element in your workouts.
Lastly, take everything you read on the internet with a grain of salt. Anyone who is absolutely sure of everything is suspect in my book. I'm working on becoming wise, but I'm finding that the path to wisdom truly is an education in your own ignorance. I am hoping that the wisdom outpaces the ignorance soon, although Socrates himself said this is futile.