Last Summer I didn't have any luck with temp agencies. I was looking to work for about 1.5 - 2 months during the Summer, but I left it a bit late and apparently they all had loads of students on their books already, so they didn't want me!
However, this year I was more successful. I only managed to register in person with two agencies but one of them is pretty good. It's called Pertemps, and as I live in Edinburgh they provide most of the temps for places in the Scottish Government, and there are loads of them. On quite a lot of the websites for national temp agencies you just seem to register online, but I'd definitely suggest that you phone them up, e-mail them your CV and phone them back again a few days after you've sent your CV. You definitely have to be the one to make an effort. I doubt Pertemps would have got back to me after I e-mailed them my CV, but when I phoned them back checking that someone had had a chance to look over my CV I was immediately offered a slot to register with them at their office.
So yeah, phone as many as you can, e-mail in your CV and follow it up with a phone call. You could try just going into their offices without an appointment but I was never brave enough to do this...