Rule #1. Never slide head first at another players feet, especially if they have the ball.
Rule #2. If you are going to crowd the goalie for a header on a corner kick, expect at least 3 other defenders getting ready to slam you. So lead in with one knee up, to keep separation from the goalie, and both elbows straight out at shoulder hight then crank them around as you are about to make contact (try to) with the ball. Keeps the defenders more docile, especially the one getting ready to headbutt you in the back of the head.
Rule #3. If you are taken down hard more than once, try to plant a knee, elbow, hand or foot on the opponents face (preferably) as you go down. That gets your point across that you know how to play the game also. (Just make it look accidental). Some players back off, many get pissed off. Either way you win. If they get mad, your in their head and typically they earn themselves a yellow card or even a trip to the bench, if not, they become too aggressive and start looking for ways to slam into you, which means they are watching you and not the ball so if you have any skill whatsoever, you can make a mockery out of them to the point of them getting pulled by their coach or the ref sending them to the showers...
Rule #4. If your coach hasn't taught you these simple fundamentals of the sport, you are playing for the wrong coach.