I have made a long list of 80+ agents/agencies from the 'Writers' and Artists' Yearbook', and the 'Guide to Literary Agents'. I will check out their websites and see if their requirements match their listings, because some of their requirements may have changed. Then I will check their submissions policy. I have already written a synopsis and covering letter, but I am still polishing my manuscript. When I'm ready I will send out 10 submissions to test he water.
I won't send submissions to any agent/agency that wants sole-agency before a deal is struck. I laughed at one agency online. Their submissions guideline said, 'We are not a beauty contest'. I won't be submitting to them.
I was told it's better to submit to more than one agent/agency. I know it's hard, but If you can hook more than one agent/agency, you stand a better chance of getting a good deal.
I remember watching a money programme. The experts advised against letting an Estate Agent have sole-agency when selling your house. That's because the Estate Agent is guaranteed their cut no matter how long it takes to sell your house. Make agents/agencies compete until you strike a deal with one of them, and then you're more likely to get a better price.
I'm not at this stage in my writing yet, I'm still polishing my draft, but I just wanted to wish you luck :)
The most I've sent out at one time was 3, and then one at a time later on. I found that it got very hard to remember what agent wants what, even with extensive lists. I think for the next time I sent my work out I'm going to send my submission to one agent per day, spread the stress out a little. :)