Yep, it's just practice rather than a trick.
What i do is really make sure I've committed the bass part to muscle memory. Then I really practice the vocal over and over. Then I practice them together until I can do it over an over. Then I work that into that part when practicing the whole song or it's context. Eventually you'll never play that bass part again without singing along.
It general, the process gets quicker and easier, some parts just require more practice, others come relatively instantly.
It's a rub the tummy, pat the head thing, once a part clicks, you can do it on demand, until it clicks it can feel unnatural.