Most of the routines that can be found online are not optimal for natural lifters, mostly because the most popular ones tend to be adaptations of famous training programs created by bodybuilders, actors, or fitness models that use PEDs. The use of steroids is extremely common (much more than you may think), and while I personally have nothing against PEDs for personal use, it is undeniable that when you use them, the optimal way of training changes dramatically. So, if you're a natural lifter, the next time that you are evaluating or creating a potential routine, go through this checklist to see if it has potential.
A very much needed disclaimer: this is not absolute. These are recommendations based on my experience and research. They will be exceptions to every one of the rules laid out below, so take it as advice and decide whether or not you want to follow it on your own.
1) Will you be training each muscle group two (or more) times per week?
A natural lifter should try to work each muscle group directly at least twice a week. The key here is the rate of protein synthesis in muscle cells. Muscle protein synthesis is the process through which your body repairs the micro-tears you create in your muscles when you exercise or make your muscles work hard. Although it is a constant process, the rate of muscle protein synthesis becomes elevated after resistance training. This is the process through which new muscle is built. When you work out, micro-tears occur in your muscle, and by repairing them you can make your muscles grow. The elevated rate of muscle protein synthesis lasts for approximately 24-48 hours after you train, maybe even 72 hours for a novice lifter. What this means is that after you hit the gym, your muscles will grow only for up to 48 hours after your training session. Because of this, you should aim to train each muscle group directly at least twice a week, otherwise you would be potentially "wasting" up to 5 days of muscle growth!
2) Will you be focusing on compound movements?
Compound movement are exercises that engage multiple joints and recruit multiple muscles, the opposite of isolation movements. Think for example of the difference between a squat and a leg extension. When you squat, you are using both your hip and knee joints, and stressing your quadriceps as well as your hamstrings. In the leg extension machine you are only using your knee joint and working only your quadriceps. The basis of every successful routine should be compund movements, such as squats, deadlifts, bench press, rows, etc. These are the fundamental excerciese that every lifter should master, not only because they are highly effective in terms of building muscle, but also because they are the ones that build real, serious strength and power. You will not grow much or gain much stength by only doing bicep curls, I promise you that. Compound movements will also strengthen your bones, tendons and ligaments, which tricep kickbacks won't do. Be wary of any rotuine that doesn't emphasize compound movements.
3) Progressive overload
The single most important workout principle for any lifter is probably that of progressive overload. What is progressive overload? It is a principle that states that in order to keep growing and keep getting stronger you have to gradually increase the stress you put on your muscles. This has been researched over and over, with studies proving the effectiveness of this training method in muscle growth and strength improvement. The reason this is so important, is that your body won´t grow unless you force it to, period. For improvements to be made your body has to be forced to adapt to levels of tension and stress previously unknown to it. Progressive overload, or rather the lack of it, is one of the main reasons why people don´t see the improvements they want to see when they start going to the gym. They try to do the exact same workout, lifting the exact same weight and performing the same number of sets and reps every time, and of course, they get discouraged because they don´t see the results they hoped for. It is only normal for your body to stay the same once it has adapted to a certain workload, so you have to constantly make your body work harder. Now, it´s quite clear that there will be workouts where you simply cannot do more than previously, but the point is to always try to do more and work harder.The way to implement this in your workouts is actually quite simple. You just have to try to do a little bit more work each week. This extra work can come in a variety of ways, such as more weight, more reps, more sets, or less resting time. Simply DO MORE each training session, and results will follow.
Weight training is an extremely complex science, and of course there is much more to the story than the previous three simple rules, but they are an excellent starting point for natural lifters trying to get stronger and bigger. Try them out and you'll see results, guaranteed.
Studies:  MacDougall, J. D., Gibala, M. J., Tarnopolsky, M. A., MacDonald, J. R., Interisano, S. A., & Yarasheski, K. E. (1995). The time course for elevated muscle protein synthesis following heavy resistance exercise. Canadian Journal of Applied Physiology = Revue Canadienne de Physiologie Appliquee, 20(4), 480–486. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/8563679
 Hostler, D., Crill, M. T., Hagerman, F. C., & Staron, R. S. (2001). The effectiveness of 0.5-lb increments in progressive resistance exercise. Journal of Strength and Conditioning Research, 15(1), 86–91. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/11708713