I have only been playing the game for two months now (real time). So far I have found it most efficient to to do three things:
1) Save your experience points for increasing mental skills. Using them to 'buy' victories in tournaments is mostly a waste until you cannot increase the mental skills anymore. An exception might be if you NEED a victory to reach a sponsor bonus, to get into the main draw of a crucial tournament, or to improve the player's ranking to meet a contract deadline. I would never do it just to earn some money unless the player was well developed.
2) Any week the player is at the training center (which I try to make about half the time), I always put twelve hours into training physical skills. Until the skills are nearing their potentials I train two hours each at footing, weight training, sprinting, bicycling, swimming and yoga.
Eventually you will reach a point where swimming and bicycling will not help you, so drop them and do three hours each on the other four. At a later point you can drop footing and bump the remaining exercises up to four hours.
At this point you will have to keep an eye on how close your speed and strength are getting to their potentials. If one is much closer than the other, than cut back on that training and increase the opposite type.
After doing the physical training I always put in six hours resting, then do some mental exercises (if I have experience points stored up). Only when that is done do I look at training the technical skills. And be sure to leave enough hours at the end of the week to rest yourself back up to near 100%!
3) Never neglect to spar when a partner is available. It is the most efficient way to improve the aggregate of your technical skills. Unfortunately, you never know which skills will improve the most (or at all).
If your sparring partner either always wins or always loses, you aren't getting much effect from your sparring, so maybe you should do physical training instead (which, like sparring, does not require an expensive coach or assistant to be present).
I frequently (meaning more than once every time I play) refer to the documentation. In this case you should especially look at the descriptions of styles of play (puncher, counter, etc.) to see what skills your player's style relies upon, and the list of 'most important skills' that Emmanuelle included.
If you find that your sparring is causing you to increase mostly in skills that don't particularly benefit your player, or if your player is becoming very unbalanced (mostly forehand, for example) you will need to hire a trainer who can help you learn the skills you need to bring yourself back into balance, or else spend much longer to learn them from just your coach.
I find that I do not have the money to hire assistants or trainers until six or (more likely) nine months into the game, and that is when I have two beginning and two advanced players. With four beginning players it is more like fifteen months. By that time their physical and mental skills should be very nearly up to their potentials. Once there, you will still need to put in maybe six hours a week at each to keep them there.
That is when I would look at hiring the +20/+25% trainers to work on their technical skills. If you can afford trainers before then, get the one who helps with mental and physical skills. Emily (the default player) can crush a player with 45% technical skills when her own technicals are around 18-20%, but her mentals are 50+ and her physicals 40-60%. It is really quite amazing (and gratifying)!
If you have any advice to share with us, please feel free to do so. (Oh, one small secret — If you are going to train physical and technical skills in the same week, or physical and spar, do the physical first. It raises some of your technical potentials, and the bigger the difference between your actual and potential rating, the bigger the increase you will get from training that skill.