When the vast majority of people do incline dumbbell curls, their head comes off the bench. Half of them do it to gawk at their awesome selves in the mirror, but the other half likely do it because they've given themselves a permanent forward-head posture by texting during walking, eating, and making sweet, sweet love.
If you're in that latter half, try sticking a rolled up towel underneath your neck and keep your neck pressed against it and your head flat against the bench. It'll automatically increase your strength.
It's an old trick I learned from Charles Poliquin and while he never explained it, I'm sure it has something to do with the towel aiding in the proper alignment of cervical vertebrae 5, which controls the biceps and traps. The proper alignment, I'm theorizing, increases your strength because it creates unimpeded and therefore enhanced neural transmission to the biceps.