I'm going to lay it out step-by-step for you. This is how I did it and your mileage may vary. I'm going write whatever comes to mind because this topic is quite extensive.
First of all, you need to believe you're ready. It doesn't matter what anyone else says, if you think you're ready, you're ready. Most of us suffer from, "I don't think my book is strong enough..." I did. But regardless of what you think about your book, you're ready when you think you're ready.
Now about the book. Yes, it helps to have a strong book? What's a strong book? Do your pictures look like what you'd find on the agency websites? No? How far off are you? Needless to say if you're on par with what you see, then your book is probably ready? Still have doubts, email me at email@example.com and I'll tell you if you're ready or not. Again, none of this makes an ounce of difference.
So assuming you A) believe you're ready and B) have a half decent portfolio, what now?
Well, hopefully you've been shooting models of the same caliber and not hood-rats off the street. I mean, in having a half decent portfolio, hopefully you've come across models that are of the caliber of the agencies you want to test for. Hopefully this means that you've shot some agency models, but just not via the agents themselves. What you do is you take the pictures of these agency-girls and you show them to the agent that you want to shoot for.
Okay, how do you know who you want to shoot for? How do I find this agent? Do I email or send them a sample of my book via snail-mail?
Okay, first of all be resourceful. Don't be a retard. Use the power of the Interwebz for crying outloud. If you can't find the name of the agent at the agency, you're a twit. Ask your friends (who are agency-photographers), do a simple search like "New Faces" and "[agency name here]", or...
Just pick up the goddamn phone and call the agency and ask, "Could you please tell me who's responsible for your New Faces division? Could you give me his/her email?"
Done. Now you have a contact. If you felt like a moron for not having thought of this yourself, you should be ashamed of yourself.
My #1 recommendation for photographers is: Stop hiding behind your computer. Pick up the phone. Talk to a real person.
Do not *I repeat* DO NOT send your agent an email with your website link assuming they'll read the email and call you/write you back. One of my agents has thousands of unread emails in her inbox. And that's a *good day* for her. Don't be one of thousands. Be one-of-one.
Assuming you're still with me. Which agency do you call first? Success begets success. Don't go calling IMG as your first agency. I can't help you if you fail. I don't even know if anyone will talk to you. Hell, I don't even know if they'd talk to me ;) Actually that's not true, I'm very persuasive and I'd force someone to talk to me, but that's beside the point.
Choose a smaller agency... preferably one that's hungrier and doesn't get as many offers for testing as other agencies.
Hopefully this agency is the same agency that signed the model you shot with. What you do now (as I mentioned earlier), is show the agent pictures of that model. Agents love seeing pictures of their models. When you show them pictures of their models you're doing two things: 1) You're showing that you are obviously on a level where the model is willing to shoot with you on her own 2) You're showing them images of someone they know so they can measure your images against existing images of the same model.
But how do you show them these images? You do this very strategically...
Email them an hour or two before you call. In your email message, make sure you introduce yourself, blah blah blah, I shot with so-and-so, here are some of the results, I will be calling shortly to follow-up...
The key points here are:
1) You tell them who you are, duh.
2) You shot with one of their models
3) Provide them your website BUT just in case they don't click, embed 2 *small* images on the email. You're forcing them to view your pics without clicking through to your site
4) You're putting the onus on yourself for calling them. They don't have to call you (they probably won't anyway)
5) You tell them exactly what you want, "to be an agency-approved photographer"...
Now, you wait. 1-2 hours should do it. No more no less. Less means they might not get/read the email. More means they've totally forgotten about you. Up to you how long you wait. Never call after lunch. They're sleepy and not listening to you. Hell don't call ME after lunch, I won't want to talk to you either :) Don't call them first thing in the morning either. They're scrambling and busy. Also don't call them just before they go home, no one wants to stay late to do something like this... send the email around 1PM and call about 3ish. There. Done.
When you call. That's all on you. I can't tell you how to be a real person. Just be you.
What I can tell you is, don't take a lot of their time. Agents are very people-oriented. They're nice because they have to be. But don't waste their time. Be polite, but straight to the point. Ask them if they got your email. They'll probably say, "No" but since you just sent it, it shouldn't be far off from the top of pile of emails. Now they'll spend 15 seconds looking at your website (but by now they've seen 2 pictures from the email already, SCORE!). They will decide if they think you're worthy of testing. They will themselves decide to send you a package of girls available.
Note: In your email make sure you tell them when you would like to test. A specific date.
You get the package (via email) and you decide on which model you would like to test. In your response please go over the details with the agent. Ask what kind of looks she (the model) needs for her book. Ask what styles the agents wants to see. Be clear. Set expectations. This is where it counts. The beginning of the relationship is when the smallest things mean the most. If you're interested in being a paid-tester, be upfront about that too. Spare no detail. Hell, I would follow up with a phone call just to verify which model you want to test and then also verbally go over the nitty gritty. Doing so accomplishes again 2 things 1) You show professionalism and that you know what you're doing and that you care about what the agent thinks 2) You're building rapport and establishing ground-rules for future interaction. The more "face-time" you get with the agent, the better.
Shoot the model. Don't screw it up. Send the results to the agent. Rinse and repeat. Shoot a few more.
From here I could go onto talking about paid-testing but I won't because far and few photographers become paid-testers.
Instead I'll go onto telling you how you get your next agency.
You don't do the same thing. Well, a lot of it is the same but once you get the first agency in the bag, the rest is easy. Why? Because agents know agents. Even from other agencies. Call up your next target. Ask to speak with the new agent and you tell him/her "I've been testing for so-and-so at such-and-such agency and I would like to shoot tests for you too"
Bam. Instant credibility. You're a shoe-in. Usually the response is, "I know so-and-so! How is he/she? How did you like working with him/her? How are the models?" Lots of times they (the agents) are actually trying to get information/news out of you so you're doing good by building rapport. Sometimes it's good news "So-and-so is doing great they just signed 20 new models!". Sometimes it's bad news, "Oh, so-and-so just left the agency because he/she got fired." If it's bad news, you probably shouldn't be bringing it up anyway. But I digress. The point is that you're approved by another agency. Hence, this new agent should approve you too.
If you have images of the new agency's girls, great. If not, just show them pictures of the other agency's girls. They (the agents) get jealous and this just might work! :)
Rinse and repeat. Now you're multi-ageny-approved!
Did I cover everything? Of course not. Don't be a twit, there's volumes more than what I've said here. But I'll be damned if this isn't the most thorough walk-through to becoming an agency-approved photographer on the Interwebz!