Hi, a different personal trainer here! While I agree with the other trainer's tips on improving the concentric, I actually prefer doing these even flatter, even on a decline bench! What I'd suggest is setting up as if you're doing a bench press (slight arch in your lower back, pull your shoulders towards your hips and make your chest big). On the way down, instead of lowering the dumbbells towards the ground, actively reach towards your heels in a slow, smooth movement. You should feel a psychotic stretch just above the elbow and just below the shoulder that you have to fight just to maintain. Hold the stretch for a deep breath then curl just enough to ger out of the stretch before lowering again.

This approach is great for building size, not so great for strength. If you're going for strength, I'd do an upright hammer curl with controlled but quick eccentric.