I love day to day supply! It takes a while to get used to it, but now I feel confident both finding and going into new schools. I try to get as much pre booked as possible as I'm not keen on the 8am phone calls but I love walking out by 4pm at the latest and having the evening to myself. I only do about 3 days a week though. It's only tough at the beginning and then schools start requesting you so you know where you're going and what to expect.
I agree, like the pre booked best. Are you working through an agency or approaching schools directly? I wrote to all my local schools with a standard application form downloaded from the county website, referees and a covering letter and got into about 5 schools as a "regular" quite quickly. I find that once you have been somewhere a few times they tend to book you up for future dates. In fact as I'm about to leave for the day I always ask if they need me again and when. I hate phone calls in the morning though.