To send to a prospective booker - you need to send something which includes images (a variety), biog, some idea of what you have done before, some reviews and something which indicates what you sound like.

Be wary of just sending links in emails to websites and such - once opened and read, an email is hard to find amongst a load of other emails and many corporate systems block myspace etc.

Personally, i prefer hard copies. If I'm interested / intrigued by what I receive, I may then look up on a website. Our system at work has rubbishy speakers which do no performer justice, so Iprefer a cd which I can listen to in the car, on the way to / from work.

Always remember that the onus is on the performer to provide. It's unfortunate but true. If you sent them something a few years ago, and they ask for it again, never say - you will have it on file somewhere. It's the performer who is looking for the booking and there will be so many other performers sending stuff in that there is no incentive for the prospective booker to look through three years of material. (They might well have moved office twice in that time) They'll just move on to the next act.