I'm mainly a short story writer. I have one unrevised novel and one unwritten one left to simmer in its planning stages, so my knowledge of agents and the dreaded book synopsis is limited to what I've read. But I do have some experience with submitting short stories and some suggestions which I'd like to share this week.
1.Professional markets: A lot of writers routinely submit to the highest paying markets first, and when they are rejected, they submit to the next highest paying, and so on down the list. This sounds rational. The problem with this is that a professional market can hold your story in limbo for months (and months) before rejecting it, and by the time the story makes the rounds of all the highest paying markets, a year or two could have gone by. My advice would be to send the story to the highest paying market where you think it really stands a chance, and then to write another, better story.
2. Following publishers' guidelines: I don't know why, but short story publications have no agreed-upon standard. Anyone who submits stories knows how annoying this is. One magazine wants stories sent as attached Word documents in standard manuscript format. Another wants attachments in rich text format with double spaces between paragraphs and no indentations. Anther does not care about the format as long as you use asterisks in the place of italics, and remove your name from the attachment. Another doesn't accept attachments at all, but wants the entire story copied and pasted into the body of an email. Another has an online form but you have to be a registered user. And then, in addition to the formatting guidelines, each publication also has specific words you are to type in the subject line of your email, or your story will be caught in the spam filter. And so it goes. Yes, I know it's a pain in the neck, but I think it's important to read the details carefully each time you submit and comply to the letter. The fifteen minutes (or thirty minutes, if you're like me!) you spend doing this will be worth it when your story sells.
3. Word count limits: Here, again, each publication is looking for something different. I think you need to take these seriously. If they say they want a maximum of 6000 words, and your beloved story has 6218 words, then either cut out 218 words, or find another market.
4. Simultaneous submissions: This means submitting a single story to different markets at the same time. Most publications want you to wait for your rejection before you submit elsewhere, which can take months. I know that some writers do not comply with this one. After all, who wants to wait four months on a response, which will almost certainly be a rejection, when the story can be making its rounds elsewhere? It's tempting, no? BUT on the remote chance that two editors want to buy your story, you will have acted unprofessionally, and you will anger one of the editors, maybe permanently. I'm sure that every writer who does this realizes that they are taking this chance. (Some publications are ok with simultaneous submissions; this will be stated in the guidelines.)
5. Proof-read: I say this, yet I myself fall prey to typos every single time I put my fingers to the keyboard. The computer will fix some of them, but not all, and I don't usually spot them afterward either, even when proof-reading. In fact, I usually don't spot them until either an editor informs me, or the story has already been published. If you're fortunate, you have someone else that will proof-read your work for you before you submit.
6. Retiring old stories: There comes a time to withdraw old, unpublished stories from the market, and in my opinion, this does not mean when you have simply run out of markets to submit to. I think that as you continue writing, some of your older stories will inevitably appear amateurish - and to the point that no amount of polishing will make you feel proud of them. If this is the case, then please put them in your trunk. I have lots of unpublished stories, and I imagine that every other writer does, too.
7. Keep writing: That's really the key, you know.
Anyway, here are links to two of the most well-known market list sites: Duotrope and Ralan's.