Make sure the chocolate and vanilla batter don't dissolve into each other while baking and stay separated.
Layer both chocolate and vanilla batter such that it's easy to make swirl patterns.
To create significant and beautiful swirls, use the pointed tip of a knife instead of a toothpick.
Do not make too many swirls so none of it is visible after baking.
After creating swirls, bake the batter immediately without resting it.