I've recently returned to folk clubs after a long absence. I find the easiest approach is to ask someone (usually whoever takes my money on the door) who the MC is for the evening and ask the MC if I can do a spot.

Sometimes I can't get a spot though if there a lot of other performers. Uusually now I do get asked though.

I'd agree that it is harder if you don't carry an instrument; I've just about been missed out a couple of times for that reason. I'm thinking of investing in a "I'm a Storyteller and Singer" badge!!!