It’s a cliché, but it’s damn right! The business of getting an agent, getting your book published and sold is definitely as hard, if not harder, as writing the novel in the first place. Similar to when you’re pregnant. You think being pregnant is bad (sickness, lassitude, ruined feet, stretch marks, galloping appetite, pan-galactic weight gain), but once the dear delight is born, you will be slogging away at a level you thought had been left in Victorian sweatshops.
One thing I’ve learnt that is crucial. You must do your research before you put one finger on the keyboard.
1. Know what sort of a book you’ve written (sounds obvious, but it isn’t always);
2. Find out which agents deal with your genre/type (Writers’ & Artists’ Yearbook, Association of Authors’ Agents, etc.);
3. Try and find the agent within a big company that deals specifically with your sort of book;
4. Read up about them on their website, the internet in general;
5. If you have any writing friends, ask their opinion.
As for the letter, there are gazillions of websites and books giving advice about being focused, businesslike and informative, but something that’s struck me as essential is some marketing advice I heard a few years ago at a business seminar from a very respected marketing guru: BE NICE!
I’m not being sycophantic ;-), just practical. Agents want books, too. No books, no business. So basically, they’re a team member in your enterprise of getting your book to the reader and it’s bad work practice to be rude to your fellow team members.