I don't know what focus systems the Canon has, but I suppose they're similar to Nikon's. This weekend, while shooting soccer, and based on the recommended AF settings I found on some website, I tried using the feature on my camera where you set the focus point and it tries to follow a particular player, but the focus point was flashing all over the screen. There are just too many players on the field for the camera to be able to track one person. So I went back to my old method, where I center the viewfinder on the player of interest and I try to keep him in the center. Also, I use the AF-ON to focus, not the shutter release. It's not a perfect system but it's the best method I've found so far.
Yes, I use AF-ON also. Some of use refer to it as BBF = back button focus. I have been using AF point expansion. Not sure its the best for shooting BB. Of course its much harder to capture certain people vs just capturing the action.