Find a mentor. The importance of this cannot be stressed enough. Find a club, and within that club, find another experienced handler to serve as your mentor. If you don't have a club, then you will have to look hard to find someone who can coach you in the sport and help bring you along. This makes Schutzhund a little easier, less frustrating, and more enjoyable, and also increases your chances for success. Having that coach there to help instruct you, guide you, spot you, and prepare you for a trial is invaluable.
Be an active student of other good handlers. Find handlers whose performances and relationships with their dogs you like. Study how they train particular exercises, how they handle the dog in a trial in each phase. Look closely at their timing, when they mark and reward and correct. When, how, and why are they marking/rewarding or correcting behavior? Talk with them about their training program (not while they are training, but after!). There is much that can be learned by watching other handlers work their dogs. When those top handlers are out training their dogs on the field, be out there with them, instead of standing and chatting on the sidelines with everyone else.
Use your resources. There are a variety of resources available, the primary of which should be your club, training director, and mentor. However, there are also training seminars, websites, online videos and training systems, online courses, books and more. We live in a day and age when information is more accessible and shareable than it ever has been. However...
...Select your resources wisely. This is crucial. Seek out the handlers who have the knowledge, experience, and success in the sport. Avoid the person who talks too much, acting as if they know everything there is to know about training, and yet have little real training and trial experience! Internet message boards and Facebook groups are full of such people, who throw out all sorts of solutions to the problem when they themselves have barely any experience from which to draw. Don't just watch YouTube Schutzhund videos, either. There are some great videos out there, but there are also some horrible ones that showcase bad or even dangerous training. "YouTube trainers" are no substitute for the real thing. Also, don't just pick a "flavor of the month" trainer or the next big guru from Europe to follow because you see their seminars being promoted. Be thoughtful about whom you select as a resource.
So who do you go to? Look for the people who have trained and trialed multiple dogs successfully, who have consistent performances and scores, who have helped others succeed in achieving their goals, and whose training and overall performances you admire.
Attend trials and listen to trial critiques. This is an incredibly educational experience! Trials expose weaknesses in a handler's training program and handling skills, and point out areas for improvement. Good judges give informative critiques that explain what could be improved in the exercise, and where points were lost. This also gives you the opportunity to see different handling styles in action.
Have a clear mental picture of what you want to accomplish. You must have a vision of what you are working toward, an end goal for each exercise. You want to teach the dog to sit, but what should that sit look like? It must be fast, straight, square, and solid. You want to teach the dog to heel, but what should that heel look like? If you have a clear picture of what you want the exercise to look like, and hold your dog and yourself to meeting this standard, then you will be less likely to teach the dog the wrong behavior or allow him to continue in incorrect form.
Don’t train alone. Have a training buddy who can spot you and watch your dog when working on things like heeling, left turns, about turns, motion exercises, long downs, and more. That extra pair of eyes is necessary to give you feedback on your timing, handling, body language/handler help, dog's position and demeanor, etc.
Videotape your training sessions. This is a huge help to see your timing, your dog's response, his position, your position, etc., especially if you don't have someone to train with at the moment. Sometimes you realize that what felt awful to you at the time actually didn't look awful, but also vice versa – that what felt awesome actually didn't look as great as you thought. Video-taping your training and then reviewing the film later will also help you pick out various handling errors. As much as some of us hate seeing ourselves on camera, filming the training session (in each phase) is extremely valuable.
Practice, practice, practice. The best way to get better is to practice. If you struggle with timing, practice WITHOUT your dog to improve this. Enlist a friend to help you, and practice marking a simple behavior, such as dropping a ball to the ground. Try marking the moment the ball hits the ground, then make it harder and try marking the moment they open their hand to release the ball. Walk the obedience pattern without your dog, and practice giving commands at all the right places, just as you would in a trial. Practice your dumbbell throws on the flat, over the jump, and over the wall outside of your normal training session. The more your practice your part, the smoother you will be. You can even practice marking and rewarding without your dog to get your motions right!
Make it your own. Don't just try to copy or mimic another handler or trainer. You might start out doing this at first to help you learn, but each trainer has a particular way or style that is unique to them, and trying to replicate every single motion or intonation can lead to frustration and a simple "parroting" of their style (and without their results). Remember, most of these handlers have the experience of multiple dogs under their belts, and have shaped their training style for years. So work on making your handling style your own as you become more proficient.
Think of it like learning to write. Initially, you copied letters and worked hard to learn how to write them individually. The attempts were clumsy, but as time went on and you practiced more, your writing improved and became more fluid. Eventually, you didn't need to copy letters or words anymore; you could write your own phrases and sentences in a handwriting that was all your own, that was distinctly you. As time went on, your penmanship became even more refined, and more distinctive. So it is with becoming a better handler. We may start off trying to copy and emulate what other experienced handlers are doing, but eventually, through trial and error, practice, and experience, our handling becomes more refined, fluid, and more distinctive. It just takes a little time and effort.