Basically what the others said, except Tachograph though I understand his sentiments.
Couple of years ago the same questions were asked by a chap who posted as Freddie Swann. I’ve seen both sides of the desk, in fact all three; in previous lives I’ve been both a TM and a recruitment consultant, now back to driving which is the same or better money for much less hassle.
Freddie asked me what, as a TM, I’d want from an agency and I hadn’t deleted the PM, so here’s the advice again for you if it helps.
I’ll say it as though I’m still doing it, easier that way. 
Honesty, first and foremost. If you can’t get me a driver, then say so on the spot. If you give me a name, and then have to change it, tell me; don’t just pick a name out of the air and then try to blag it later, it’s too damned obvious. And don’t send me a muppet. Personally I’d sooner pay 50p/hr more and get someone reliable, clean, willing and competent; but that’s just me. Remember, most TM’s get a different agency touting for their business every day, and they know ■■■■ well the only thing that might be different is the price and the level of service.
Other bits which help; brief your driver properly. Ask me if it’s a possible night out, any handball, number of drops and roughly where it’s going. If I find out that I’ve told you something important and the driver genuinely doesn’t know about it, you’ve failed. Drum into your drivers that they MUST bring their docs ( both bits of licence, DCPC etc) with them, it’s bloody annoying having to chase up faxed copies.
Don’t pester me with speculative phone calls. And don’t you DARE ring me twice in a day when I’ve said no the first time…honest I had one guy do this to me three days running and I ended up phoning his boss to complain. But if you happen to have one of my favourite drivers available next week, and you ring me to offer me first dibs on him, I appreciate the thought.
And most important of all… if you tell me you’re going to ring me back in 5 minutes, ring me back inside four and a half. The first time any agency fails to return a call when they promise to do so, is the last time they get first call for my business.
i know a lot of this is basic, and I’m also aware it’s stuff you probably know already; but the best relationships I had both with agencies and as a consultant with TM’s, were those where both sides kept it simple, honest and straight to the point.
I wish you luck, and will offer you one piece of advice which has always stood me well. That is, what a candidate’s CV does NOT say is far more revealing than what it does. 
Admittedly things have changed a bit since I was doing the job but the basic advice remains the same. Look after your good drivers, and they will look after you.