1. Find out who to contact.
In order to have the best chance of getting an agent, you need to know who is out there. Before I got an agent for The Ex-Factor, I bought a copy of The Writers' and Artists' Yearbook (published by A and C Black), which costs just over ten quid. It includes the names and addresses of just about every agent worth contacting, as well as many who aren't! For a more condensed list see http://www.andreasemple.com/agentsdirectory.htm.
2. Be a tart.
Agents like to feel special. They like to feel that they are the only agent in the world and therefore want to be the only one you contact. It's a nice idea, but as you will be waiting up to 3 months for a reply it isn't realistic.
Unless you plan living as long as Yoda, your best bet is to be a complete tart. Send to as many agents who are relevant as possible. This is controversial advice, but I'm afraid it's a lot more time efficient. That's what I did. I sent to twenty, and out of that twenty, ten were interested in my novel. Obviously, if like me you get more than one agent interested you are going to have to reject potentially very good agents. But believe me, it feels a lot better sending rejection letters than receiving them!
3. Become a brand.
What some people tend to forget is that the book world is an industry. Agents and publishers are business people and want to be sure that any author they take on can be marketed. In other words, you are not just selling your book - you are also selling yourself.
The trick is to think about yourself as a brand. If you were a product (and that, alas, is how publishers and agents see you) what would your key attributes be. Don't burden agents with your CV or your A level results - stick to a couple of paragraphs in your covering letter, and make sure you sound like the sort of person you'd like to meet. Oh, and make sure you back up everything you say with examples or it may sound a bit hollow and big-headed. For instance, if you have had anything published before mention it along with the sort of response it got.
4. Send the first three chapters.
Most novels are between 70,000 and 120,000 words. Agents simply do not have the time or the desire to read that many words from a complete stranger. Your best bet is therefore to send the first 10,000 words, or three chapters to give them a taste of your novel.
5. Write a synopsis.
As well as three chapters agents will also want to see a synopsis or overview of your novel. This should be about two or three pages long and should be as well-written as the novel itself. When writing the synopsis, try and think about what you would write on the back cover of your book in order for people to read it.
6. Get endorsement.
If you can get another author or someone well-known to read your novel, or even a short extract, and provide a quote you will have a definite head start. For instance, I managed to get a comment from Lisa Jewell (who I had never met) simply by contacting her via email. The fact that she loved the novel meant that agents were going to take more interest in me.
7. Check for mistakes.
It sounds obvious, but any punctuation or grammatical errors can prove to be the kiss of death for your manuscript. Agents are generally quite a sad breed who consider misplaced commas to be a crime against humanity. So just bear it in mind and be as anal as possible.
When I first sent out my manuscript it didn't get anywhere. I looked at it again and I realised I had spelt 'synopsis' like 'syponsis' - d'oh! I moved the letters around and got a much better response (or reponsis!)
8. Make them laugh.
If your writing can create a physical reaction - laughter, tears - it is going to stand a very good chance of getting taken on. Even in your covering letter, a bit of light-hearted humour wouldn't go amiss and you should certainly try and avoid being over-formal.
9. Chase them up.
Agents respond to pressure. If you haven't heard after a month it is always worth sending a polite email to check that they received the manuscript. If any other agents are interested, tell them that as well.
10. Don't send in March or October.
These months are the busiest for agents as they contain the London and Frankfurt book fairs respectively. Also, try and make sure your manuscript arrives towards the start of the week if you want a speedier response. Christmas and other major holidays are also good times to avoid as agents are often on holiday and will have a pile of manuscripts waiting for them when they get back.