David it is definitely worth joining. Just go to the PRS Website and fill in the form,

Then you can fill in online forms to register each of your songs, and also any arrangements of traditional works you may make.

Festivals and larger gigs that play in venues listed as concert venues will ask you to fill in a green form (which then goes to PRS), and you also, as treewind says, send in a spreadsheet with a 'generic' typical set list, together with a list of other gigs played (no matter where) for all the others. So no gig goes unpaid. (Once, you could only send in a form every 50 gigs, then they dropped that, but now there may be a limit again but I'm out of touch so you need to check).   

I stopped touring two years ago and get very little radio play at the moment but still get a healthy cheque every quarter.

Tom