It is critical to never move the hand or arm that delivers the reward until after the mark. I've already briefly talked about this but here, I get into more details on why it is important NOT to move the hand or arm that delivers the reward.
Dogs are visual animals. They will watch you all the time but very naturally, they won't listen to the words that come out of the mouth. The obvious fact is that they don't understand what it is we're saying.
Instead, your dog reacts to physical cues. He reacts to how your arm moves when it presses the clicker. This is his cue to knowing that he's going to be getting a treat. While he may take into consideration the sound of the click, his primary focus is on how you move.
You need to be careful of giving the reward at the same time of the click. Your dog won't register the connection between the two. He will be too caught up in the reward to recognize the click that bridges the gap between the behavior and the reward. Chances are, if you are rewarding the treat at the same time as the mark, then your timing is off in the first place.
People will naturally mark the behavior and give the reward at the same time. Even reaching for the reward at the same time as the mark is a physical cue your dog can sense. If you want him to start understanding the meaning of the click, then you need to keep that rewarding arm still until after a second or two. This is a very simple concept that can be easy to learn if you develop the right habits in the beginning.
To improve your timing, you can videotape your training session and study it. This is a great way to looking at your actions from an outside view and finding things that will fix your training. Not only are you making things easier for your dog to learn but you are also making it easier for yourself in the long run.