If you use the Accor hotels (Novotel, Ibis etc), you can cancel them up to 4pm on the day, so although you won't be able to book the super-cheapo (non-refundable) rate you should still be able to get a room reserved enough in advance to get a reasonable price, and you can just bin it if you decide not to bother. Ibis aren't half bad for a budget chain (although like everything else, they vary).
Have you decided on a regular day of the week for incalls which you can advertise? I know there isn't really one day that's reliably busier than any others, maybe if you had a 'routine' people could get used to. I don't really take same day bookings, but like Simone said, if you want to try it get your room organised and then throw everything you can at it - even if you're only on AW there are plenty of extra features you can use, and even though it'll rack up the costs, if you decide the date in advance I think some of these are cheaper too.