If you don't have a spanner (a tool with two screwdriver blades, the distance between them being adjustable), I have had good luck using a small tool like a jeweler's screwdriver or a dental pick (I prefer the latter--and no, I'm not a dentist) to tighten the ring. I alternate between the two slots found in most retaining rings--going back and forth, giving first this one a little nudge, and then the other--and then back again. I do this with a very light touch, almost no force at all, while keeping a light pressure towards the outside of the ring, to ensure the tool won't slip and scratch the glass. I just snug the ring up. I don't try to cinch it down to the greatest possible tightness.
“There is hardly anything in the world that some man cannot make a little worse and sell a little cheaper, and the people who consider price only are this man's lawful prey.”