I'm new to querying, and if you could see my query letters in order of submission from first to latest, you'd notice a huge difference. No results yet, much too soon. But here are things I've learned already and perhaps they'll help you who are following behind me in the writing adventure.
1. Use Query Tracker contact management software for querying agents and publishers. The basic version is free, but for only $25 per year you get some advanced search features I am already using. My husband offered to whip me up something on Excel, but he can't give me a list of agents seeking manuscripts. Query Tracker does that, keeps track of your query's progress, and much more.
2. Read agents' websites. Even though Query Tracker and similar sites such as Manuscript Wish List may include basic information on what the agent is looking for, I discovered that there's usually more on the agency website. Plus you then sound more intelligent when you mention this in your letter.
3. Look at authors the agency represents, and figure out why your novel would be a good fit. Include that insight in your cover letter.
4. Print out Submission Guidelines for each agent. They'll be different, believe me. Follow them exactly.
5. Keep track of the different versions of your hook, blurb, and synopsis and what you send to whom. Some agents want two paragraphs. One agent I queried asked for a "detailed synopsis." I still don't know exactly what that means, but I send him 1000 words and the 50 pages of manuscript he requested. At first I thought I could just use a template and change the name in the salutation, but that's what agents don't want and will get you culled in a hurry.
No doubt in another five days I'll have five new insights. But I'm betting you readers will have even more. I will reply to all comments.