I haven't used radio for domains or my own business, but have run campaigns for nightclubs, where we have had drive time DJ's plugging the event, and running adverts. Always local radio too, so some of this may not apply to national.
Its worth checking their show time demographics, to see which shows your intended customers are likely to be listening. Its no use running your ads for child minders at 3pm since your likely customers are on the school run. Its also no use running adverts for your whacky waving inflatable arm flailing tube man during the pop charts or question time.
Have a listen to the shows around the time you're thinking of running it, and check if you think your targets will be listening, Compare what you hear to their demographics.
As for the money. Work on 1 month, drive the price down, imply ongoing usage and push down more, then once you've hammered the price, pull out the offer to sign up for 3 months for a bigger reduction. Negotiate, Negotiate and build some rapport, they WANT and NEED your business, so push hard.
Oh ask about last minute slots, quite often the DJ will need a filler because somethings come up or somethings delayed. These slots are usually cut down adverts, you've prob heard them yourself, the main advert is like 20-25 seconds, then you hear a cut down version which is like 10 seconds, these are often fillers (I suspect some are legit adverts), and you can get them way way cheaper, even free to keep your business.