I have always written for TV or stage, but recently I managed to purchase some really good recording equipment, for other projects.
Does anyone have any tips or hints about writing for radio, where the gag is all verbal and there is nothing to use as a visual aid?
Or help in creating a story for the radio?
Let me know!
I think main thing is to set the scene in the first line of dialogue and use sound effects for maximum effect. For instance, if you're in a train station, use a platform announcer (possibly making a silly announcement) to set the scene or even just the noise of a train.
You could have someone at the ticket office to set the location and plot up:
WOMAN: (UPSET) Ticket to Manchester please
SALESPERSON: Return?
WOMAN: No, I'm never coming back...
That sort of thing (well, that written a lot better!) gets us interested in the story and sets up where we are and what's going on.
Also, don't 'see' the lack of visual medium as a failing, but an opportunity to paint a picture for the listener; you can do all sorts of stuff on radio that would never get made on TV due to costs.
Don't know if that's the sort of thing you meant but hope it's some help.
Dan
It's also bearing in mind that with radio you can be anywhere and create anything, so it helps to stretch the listener's imagination to the limits.
And you can be naked all the time on radio doing what you want and they don't know! 
Fancy helping us put this radio sketch show together?
yeah i dont mind, that would be really nice, thanks.
send me some information and i'll join in!!