Stay healthy
Being healthy is essential for doing anything properly in life. Both your physical and mental abilities require a certain amount of health. Therefore, you should listen your body and eat good foods that contains a wide variety of nutrients. Eat enough, but avoid exaggeration. Manage to always have good quality sleep. This aspect is often neglected, but essential. Avoid chronic stress. Keep a good hygiene, but manage to challenge your immune system regularly. Feel what make you weaker, and avoid it, unless you train to become immune to it. Per example, cold weakens your immune system, but by bearing it regularly and compensating with good food, good sleep, and warmth after you're done, it won't make you sick, and you can get good benefits from learning to be cold resistant. On the other hand, I'd avoid alcohol completely, as it haves various very negative effects on your body that you cannot (as far as I know) get immune to. You can increase your tolerance to alcohol, but it's very specific and only useful if you have to drink anyways.
Train for efficiency
First, you have to know why you're training for, and train accordingly to that purpose. You can have many different goals. Mines are simply to be ready for all situations, and always find my way out of it. Surviving by overwhelming anything that can go in my way. I think that if you train with that purpose, it can give you good results in everything you do, be it sports, martial arts, professions, general fitness or getting big muscles. I also think that you should train motions, rather than specific muscles. Thinking about specific muscles too much leads you to forget that muscles are almost never used alone, always work in chains, and have a particular range of motion that's almost always a good thing to improve. But you can work in isolation to correct weaknesses. There are also particular abilities that can be more important than others. A good example would be strength, that leads to ease every effort you make, and is essential to develop other qualities.
Learn to never give up, then learn to give up
You should be training all your life long. Unless you train solely for a career, there is no reason why you should ever stop training. But like for training, life isn't always easy. Learning to never give up in training helps to prepare you to overcome every challenge life can throw at you. It is a mental training. You shouldn't always train your mind in every training. But make sure you're ready, and feel when you need to train your mind. You can decide to meditate, run 100 km, do a high intensity training that makes you puke or pass-out, do something that scare the shit out of you, fight, inflict yourself pain to learn to manage it, take a frozen shower. Or simply going to train every time you're supposed to, when you dislike it, and make a healthy habit out of it. But whatever you do, do it well, and never give up. That's simply how you learn not to give up. And after learning to not give up, you have to learn to give up, to help you, at the end, to endure what you might face by not giving up. Giving up can also eventually save your life, or save your mind from very awful suffering.
Training sessions never end
Once you stopped doing exercises, you're never really done with your training. You have to recuperate. You have to stretch often, and keep a good blood circulation in our body. You have to keep eating properly, to sleep well. And you have to do everything the best you can, seeking the highest quality you can achieve. By doing that with everything, you learn to do that in your training as well. And while you do something else, you can also seek the most efficient motions, and find challenges, which leads you to develop better qualities. I know it sounds cliché, but masters in kung fu movies are right: "Everything is kung fu."
Training should give you more independence
At the end, your training should lead you towards a greater independence, by improving what you're able to do, giving you a better knowledge of yourself, and teaching you to face your fears and to deal with the unknown. It should teach you that you can rely on yourself as well as some of your limitations, and how you could push them further away or not. It's why you should understand your training, even if you are being trained by someone else. If something ever happens, you should be able to keep training yourself, and even if you want to keep being trained, which is not necessarily a bad thing, understanding your training will help you and your trainer to give you trainings of a better quality. And, let's face it, there's less things that can stop an independent person than a more dependent one. You should also make in sort not to be dependent of your training. Don't ever say you can't train because you can't go to a gym, or because you don't have a trainer. You can always find a way to train, and that is part of making sure your training is also independent.
Train to be complete and useful in all situations
As I already said it, I see training in a really practical way. You have to carefully choose how you train, and why you train. I personally think that specialization can lead you to have overall more handicaps than being pretty good at everything. It's sometimes the case with sports. Here's two examples: a high level marathon runner, and a high level powerlifter. Both sports brings great benefits, but at the expanse of other abilities (if you go far enough in specialization, of course). Highly specialized marathon runners doesn't have any power, and most of them don't have any upper body strength. They get incredible times in marathons, thought. But what about the rest? Highly specialized (especially the heaviest ones) powerlifters can also develop problems related to their sport, and if one can squat over a thousand pounds (with gear), he might as well weight 300 pounds, and that is pretty tough on the heart. Plus, there's good chances they'll end up with a serious lack of mobility. An other aspect of training to be complete is identifying our weaknesses, and correcting them so they don't slow us down anymore in the future. Correcting our weaknesses also leads to a greater independence. And don't forget to do good quality work. Don't try to mix everything either, or you'll just butcher pretty much everything you're trying to train.