Below is a list of query tips I have gathered from agents. I am, of course, going to elaborate on each one, but I have bolded the list so you can skip past my comments if you would like to. I also apologize for not being able to give credit where it is due, but I have read so many query tips that I can't remember where I got each one.
1. Don't query too soon
Make sure your novel is in the best possible shape before sending out your queries. I know, it is exciting, you love your book and you want to get it out there so everyone can love it too, but there is a lot of competition out there, so you need to make yours stand above the rest by making it awesome.
2. Don't put "Imagine if..." in your query
Show don't tell. I don't have to give credit for this one because almost EVERY publishing professional has said it at one time or another. A good writer should be able to paint a picture so readers don't have to imagine.
3. Don't blast everyone at once
This is very important. I send mine in little groups at a time, testing alternate queries and making changes as I go. This is a process and the more you do it, the better you'll get. Besides if you send out one mass blast and everyone rejects you, than that novel is finished.
4. Keep your letter professional
There may be differing opinions on this one. I have seen succesful queries that are creative, witty, funny, etc., but I have received a better response from my letters that are straight up business letters, like a cover letter for a resume. Also, like a resume cover letter, I mention in the first line where I discovered said agent and why I am querying them.
5. Only query agents who represent your work
I have seen differing opinions on this one as well. I think Janet Reid said to query everyone - because you never know, but Nathan Bransford mentioned yesterday that he's experiencing a query deluge, and probably a good chunk of those are for titles he doesn't represent (although from his blog it appears he'll look at anything that tickles his fancy) But not all agents are the same. Best bet is to review their submission guidelines on their website and stick to them. I also wonder, as a querying author, if mine gets lost in the slush pile because agents are just so inundated with proposals. At my job, I have to filter through about 100+ emails of SPAM every day and I've often deleted a relevant email just because it is surrounded by junk. Do you want your email to be classified as junk?
6. Only query agents you want to work with
If you don't feel an agent will be a good match for you, don't query them. Enough said.
7. Don't respond to rejections with something nasty
I don't even know why I have to list this, but I see in a lot of Twitter feeds and blogs that agents still receive ridiculous things from authors: responses trying to change their minds, telling them how they're missing out on a best seller, etc., etc.. NO! The most important thing in business is networking and the first rule of networking is don't burn bridges. You don't want to say something in haste and get labelled as crazy writer, someone difficult to work with, someone unprofessional. Agents talk, and once something is in writing it can be sent from California to China with the click of a mouse. If you are really irked about something, go out with a friend (not associated with the publishing industry) and vent about it over a beer.
Okay, that's all for now, but if anyone has something to add or something to dispute, please feel free to post it in the comments.