Karen Sandler (kmsandler on QT) has signed with agent Matthew Bialer of Sanford J. Greenburger Associates.
Once you've done that, study the databases like QueryTracker. Make sure you focus on agents who represent what you've written. If your book is a genre they say they don't represent, DON'T SUBMIT TO THEM.
Go to the agency website to double check how they want their submissions. Sometimes the databases don't keep up with recent changes (e.g., agencies transitioning from snail mail only to an e-mail option).
Make sure your query is squeaky clean grammar and spelling-wise. If you can't spell or write a coherent sentence in your query, an agent will likely get the impression you can't write a book either.
Only include what's pertinent in a query. If you've written a thriller featuring a retired cop and you are a retired cop, that's pertinent. If it's a mystery centered around Napa Valley's wine country and you're a vinter, that's pertinent. However, if you're a tournament bridge champion but you're writing a fantasy novel featuring elves and dwarves, the bridge expertise is most definitely not pertinent.
Also, don't take the rejections personally. Quite often, despite all your research, your book is just one that the agent doesn't feel they can do a good job representing. You want an agent to take you on because they're crazy-enthusiastic about your work, as opposed to one who feels so-so.