I caught myself doing this at my sonís roller hockey game the other week (as both an assistant coach and a sideline dad) and I thought to myself, "I wonder what he thinks when I yell instructions to him." So I asked him, "You do know that when I yell instructions to you, I'm not yelling at you, I'm just trying to help?" He responded, "I know that Dad, but you could wait and just tell me when I come over to the bench or after the game." And you know what, he's right.
The only person giving instructions to the players should be the head coach or the assistant coaches. We often forget (or have never experienced) what it's like playing in a competitive atmosphere. It's hard enough running, getting where you're supposed to be and playing your best, let alone having to listen to three different people yell three sets of instructions that you're not doing or should do (and usually never the same things).
Next time you're going to yell something to your child, try yelling, "Good job, keep up the good work" and leave it at that. Hopefully the parents around you will follow your lead. I guarantee your child won't tell you after the game, "Geez Dad/Mom, I missed you yelling instructions to me during the game."