I had a really hard time getting used to manual BPs as well, and even now it's not something I'm great at since I usually use the monitor or doppler for BPs.
I found it helpful to have a baseline to go from... have someone else take the BP first (or use an electronic cuff), then you have an idea where you'll first hear the beat. That way, if the systolic was 120, you can pay extra close attention between 130 and 110, and you might have a better time learning to identify the first sound.
Also, as others have said, make sure you palpate the pulse first so you are definitely going to hear it when you place your stethoscope there. I usually hold the patient's arm with my fingers under the elbow (holding it straight) and my thumb holding the head of the stethoscope in place.
A fellow student can be a lifesaver... In first year, I checked one of my classmate's BP about ten times a day until I felt like I knew what I was doing... she was really patient, and kept her arm nice and still