It's definitely not easy for most people, myself included.
I've found that 1st off, I have to have the bass line and vocal parts down 100% separately. Once I have them down, I try to combine the two and find where I stumble. Then, as someone else said break down those sections where you have issues into smaller parts and try and find a way to get them nailed down through repetition.
It's another one of those "there's no shortcut around practice" things.
Agreed, it's something I need to integrate more into my regular practicing, even when not called for so that at least it becomes more natural and less forced.
Gotta admit, I've cheated sometimes and modified my bass line so that it corresponds more to the vocals. Not proud of it and only did it when it wasn't super obvious, still, not optimal.