Here are some thoughts from a blog by Linda Parelli:
…The key is to put it all in balance – big rewards for big tries, small rewards for small tries. Sounds simple doesn’t it? The hard part is expecting the appropriate level of response, and I say “appropriate,” because how much response your horse can give is directly proportional to his level of education and the ability and confidence you have to ask for it. For example, you don’t want to ask a green horse or a worried horse for more than he’s able to give.
Pressure and release is the cornerstone of horse training – it’s a simple as that. But how you ask, and when you release is where the art of horsemanship really is.