public class mod_MyModName extends BaseMod {
    public void ModsLoaded() {
        RenderPlayerAPI.register("MyModId", MyModRenderPlayerBase.class);
        ModelPlayerAPI.register("MyModId", MyModModelPlayerBase.class);
    }
}

public class MyModRenderPlayerBase extends RenderPlayerBase {

    public MyModRenderPlayerBase(RenderPlayerAPI renderPlayerAPI) {
        super(renderPlayerAPI);
    }

    // one of my hook functions
    public void renderPlayer(EntityPlayer entityplayer, double d, double d1, double d2, float f, float f2) {
        if(myModIsActive) {
            // my code
        } else {
            super.renderPlayer(entityplayer, d, d1, d2, f, f2);
        }
    }

    // another of my hook functions
    public void loadTexture(String s) {
            if(myModIsActive) {
            // my code
            if(callLoadTextureOfRenderPlayer) {
                renderPlayer.localLoadTexture(s);
            } else if(callLoadTextureOfRenderLiving) {
                renderPlayer.superLoadTexture(s);
            }
            // my code
            if(ownTextureToLoad != null)
                super.loadTexture(ownTextureToLoad);
            else if(dontLoadTexture) {
                // my code
                return;
            } else {
                super.setRotationAngles(s);
            }
        }
    }
}

public class MyModModelPlayerBase extends ModelPlayerBase {

        public MyModModelPlayerBase(ModelPlayerAPI modelplayerapi) {
            super(modelplayerapi);
        }

        // one of my hook functions
        public void render(Entity entity, float f, float f1, float f2, float f3, float f4, float f5) {
            if(myModIsActive) {
                // my code
            } else {
                super.render(entity, f, f1, f2, f3, f4, f5);
            }
        }

        // another of my hook functions
        public void setRotationAngles(float f, float f1, float f2, float f3, float f4, float f5) {
            if(myModIsActive) {
                // my code
                if(callSetRotationAnglesOfModelPlayer) {
                    modelPlayer.localSetRotationAngles(f, f1, f2, f3, f4, f5);
                } else if(callSetRotationAnglesOfModelBeped) {
                    modelPlayer.superSetRotationAngles(f, f1, f2, f3, f4, f5);
                    // my code
                }

                if(hideHead) {
                    modelPlayer.bipedHead.isHidden = true;
                } else if(mirrorBody) {
                    modelPlayer.bipedBody.mirror = true;
                    // my code
                } else {
                    super.setRotationAngles(f, f1, f2, f3, f4, f5);
                }
            }
        }
}