Making sure to work both heads of the bicep.
True about all the 'genetics' replies. If you have a short bicep, the peak will appear taller. I always have begun my workouts with a barbell curl, very wide with index finger on the rings and arms rotated slightly out. I then move on to DB curls keeping elbows tight to sides and curling up and turning the wrist at the top of the movement (as if thumb would point more toward the floor). Sometimes DB curls seated, or standing and sometimes a drop set of four weights in descending order.
Preachers are excellent and you can vary the grip on these too. Hammer curls...Cable curls (I would always do these with a straight bar because I could put 100lbs on a cable exercise and it didn't torque my elbows and tendons like DB's can).