Read agent blogs. Rachelle Garnder, Nathan Bransford (former agent), Bookends LLC, and more. They give lots of tips. I am currently querying here and there, but have so far been rejected. I was asked for a partial after a pitch at a writers conference. Pitching in person is great because you get instant feedback, but it’s scary. Still, well worth it, if you get the opportunity.
Also, check out Agent Query Connect which is a great place for all things query!