General aspects of skill
How efficient or 'clean' your technique is.
Your mechanics are a measure of how well 'programmed' your body is to the motions of the skill. Notice that your mechanics can improve not only through reptition but by visualization and observation too. If you watch somebody execute the instep drive, for example, you learn subconsciously by relating to his movements.
How frequently you can execute the technique with success.
The difference between a pro and an amateur lies in consistency. Techniques which you can pull off 2 in 5 tries, the pro is expected to succeed in 4 out of 5
Consistency can drop temporarily if you don't rehearse on a regular basis and when you're tired (physically or mentally). High level teams conduct a "technical warm-up" for about 20-30 minutes prior to every match. Their players rehearse different techniques and get as many repetitions as they can in that short, but critical amount of time.
When you first start learning a new move, your mechanics will rapidly improve. You'll then reach a point where your subsequent training will mainly help your consistency. If you feel that you lack consistency, you're probably still in first, "mechanics" phase. The two curves meet at the point where you've 'learned' the skill. When you keep practicing after that point, you're only improving your consistency.
You must have a routine if you want to improve. Never go out to the field and then say to yourself 'what shall I work on today?' Set your goals in advance and when you get to the training pitch, start working on drills right away. Casually messing around with the ball is easy. If you want to improve you have to push yourself out out of your comfort zone.
Without setting goals in advance, you are not really training. You need to break up your ultimate goal, into small achievable steps. You will feel gratified after making each step towards your goals and you'll know that you are making progress. Again, the key is to set both long and short term goals.
These goals you set should be specific and measurable. Instead of saying "I want to run longer" you should say "I want to be able to run 2 miles in 12 minutes in a month's time".
What to train
Working on many skills at the same time is not focused training and it will slow down your progress. The most efficient way of training is to divide and conquer. Set your goal so that it's focused on one specific skill. Work exclusively on that skill for 5-20 sessions (or until you have achieved your goal) and then move onto another skill.
How much to train
The lenght of your training sessions may vary anywhere from an hour up to two-two an a half or possibly more. Yet more is not necessarily better. It depends on the intensity of your workout. If you are doing fast-paced drills or sprinting your training sessions won't last very long. On the other hand, if it's your off-season, then you may even do two sessions per day.
Remeber that your body must be able to recover, so that will also affect the total number of hours you put in.
Highschool players want to set records, to get a great touch on the ball and become so skilled that college coaches will be looking for them.
The bottom line for improving soccer ability is thousands of quality touches (or repetitions) on the ball. Dribbling, feints, shooting, and control touches. It all boils down to touches. Add in some speed (simulated pressure) to your training and you'll get to the next level quickly.
Find a rebound wall to work with or buy a rebound net. A simple wall is ideal and two walls with a corner is even better. Be creative and apply different parts of your body. Use your inside, instep and outside of the foot. Practice all sorts of traps with your chest, thigh or head. Don't be stationary, learn to control and turn with the ball at the same time.
Try using feints and then shoot. Hammer the ball for weeks/months, until you are comfortable shooting and volleying from all sorts of angles and positions, with both feet.
Feints and fakes
Pick a few feints that you want to master and concentrate on them. 99% of all top soccer players have only a few moves but they work every time. Always try to visualize the situation where you would use this move on the field and how the defender would react.
Regularly work on your juggling skills. The point there is to get touches and improve your 'feel' for the ball. Try to challege yourself by flicking the ball over your head and then continue juggling or try to run and juggle at the same time. Always push your skills to their limit, that's the sure way to improve.
Contributors: expert, jdefoe, Icy