Sorry to say but the only way I have found to get riders attention is to scream. I try to be polite and just yell "rider up" but that never works. Either the rider is f*cking deaf or just stoopid, but either way saying nor yelling never works.
If your on the trails you have to keep your ears open for other riders saying "rider up". It's way too much of a risk for both riders if there is no communication.
Following my buddy one time as he tried to pass a slower rider on CIU, yelled "rider" for 30 seconds with no avail. Finally tried to pass and the rider veered into my buddy and they both ended up in a ditch.
Everyone just needs to open their ears…
The ego boost you get from screaming at women learning to ride DH on intermediate learning trails must be really satisfying. Do you heckle children too?