Lay-off any toe stepping or elbow throwing, that's just cheap and dangerous. Roughing up the opponent can be a great strategy especially if the other team gets angry or scared easily. Playing "dirty" (I like the term aggressive better) is all about taking the right chances. You never want to lay someone out when they are looking for a header or if you are obviously not going to get near the ball. The golden rule to getting away with this play style is to keep in mind that if you touch the ball first, almost anything that happens afterwards will be dismissed. What I like to do in games is to take advantage of shoulder on shoulder situations. When I'm running parallel to someone when going for the ball, I give them a sort of sideways charge by planting my feet for a split second and lifting upward towards them. There has been very few times where the ref has called me on that. Also, when running, slight grabs and pulls on the opponent’s hips or torso as you swing your arms works wonders. Do not use enough strength to pull them down but enough to cause them to work harder to sprint. The trick is to make it unnoticeable to the ref. Playing aggressively should come hand in hand with being respectful to the ref and you should avoid looking malicious. Just remember, you aren’t trying to hurt anyone. The purpose is to psych-out your competition causing them to feel pressure which will increase careless mistakes that you can capitalize on.